Chiton crinitus: Pennant, 1777 - Pennant 1777: 71, pl. 36, Figs. 1, A1.

non Chiton crinitus: Sowerby II, 1840a - Sowerby II 1840a: figs 88-93; 1840b - Sowerby II 1840b: 2.

Chiton onyx: Spengler, 1797 - Spengler 1797: 95; Kaas, 1981 - Kaas 1981: 220, fig. 6.

non Chiton onyx: Morch, 1870 - Mörch 1870: 113.

Chiton fascicularis: Brown, 1827 - Brown 1827: pl. 35 fig. 8 (not fig. 5); 1844 -Brown 1844: 65, pl. 21 fig. 8 (not fig. 5); Sowerby II, 1840a -Sowerby II 1840a: figs. 87, 87a; 1840a - Sowerby II 1840b: 1; Sowerby II, 1859 - Sowerby II 1859, pl. 10, fig. 5; Forbes & Hanley, 1849 -Forbes and Hanley 1849: 393, pl. 59 fig. 5; Hanley, 1855 -Hanley 1855: 15; Jeffreys, 1865 - Jeffreys 1865: 211; 1869 - Jeffreys 1869: 197, pl. 55 fig. 3; et mult auct.

non Chiton fascicularis: Linnaeus, 1767 - Linnaeus 1767.

Chiton fascicularis var. minor: Philippi, 1836 - Philippi 1836: 108.

Acanthochaetes vulgaris: Leach, 1852 - Leach 1852: 229, pl. 10 fig. 8.

Chiton fascicularis var. attenuata: Jeffreys, 1865 - Jeffreys 1865: 212.

Acanthochites aeneus: di Monterosato, 1878a - di Monterosato 1878: 147.

non Acanthochites aeneus: Risso, 1826 - Risso 1826.

Acanthochites (ton) adansoni: de Rochebrune, 1881a - de Rochebrune 1881a: 44; 1881b - de Rochebrune 1881b: 116; 1881c - de Rochebrune 1881c: 238, pl. 17 figs. 9a-b; Pilsbry, 1893 - Pilsbry 1893: 13, pl. 8 figs. 33-34; Thiele, 1909 - Thiele 1909: 43, pl. 5 figs. 69-73; Bergenhayn, 1931 - Bergenhayn 1931: 28, pl. 3 fig. 81; Leloup, 1968 - Leloup 1968: 61, figs. 3-7, 11, 14.

Acanthochites (tori) bouvieri: de Rochebrune, 1881a - de Rochebrune 1881a: 45; 1881b - de Rochebrune 1881b: 117; 1881c - de Rochebrune 1881c: 239, pls. 17 figs. 10a, b; Pilsbry, 1893 - Pilsbry 1893: 13, pl. 3 figs. 65-66; Thiele, 1909 - Thiele 1909: 42; Leloup, 1968 - Leloup 1968: 62, figs. 4-7, 14.

Anisochiton (Acanthochites) fascicularis var. violacea: Pallary, 1902 - Pallary 1902: 29.

Acanthochites fascicularis vars var. lutescens, cinnabrina et fusca: Dautzenberg & Durouchoux, 1906 - Dautzenberg and Durouchoux 1906: 15.

Acanthochitona crinitus: Winckworth, 1926 - Winckworth 1926: 15; 1932: 218.

Acanthochiton fascicularis: Leloup, 1936 - Leloup 1936: 3, fig. 3; 1968 - Leloup 1968: 60, figs. 1-6, 8-11, 13-14 (bibliography); et mult. auct.

non Chiton fascicularis: Linnaeus, 1767 - Linnaeus 1767.

Acanthochiton gracilis: Leloup, 1968 - Leloup 1968: 74 (ex parte).

non Chiton gracilis Jeffreys, 1859 - Jeffreys 1859.

Acanthochitona crinita: Kaas, 1985 - Kaas 1985: 588, Figs. 7-50.

Type material. Neotype: RSMNH 1978.052.02601, Royal Scottish Museum of Natural History, Edinburgh, Scotland. Designated by Kaas (1985) (material not seen).

Type locality. Scotland, sea near Aberdeen (Pennant 1777); Scotland, Monach Islands, North Uist, 57°31.5'N, 07°38.5'W (Kaas 1985).

(See Jones and Baxter (1987), Dell’Angelo and Smriglio (2001), Bonfitto et al. (2011), Mitov (2015), Schmidt-Petersen et al. (2015) for additional descriptions that pertain to A. crinita s.s.). Small to medium-sized (in examined material, BL: 8–18 mm, BW: 4–10 mm, BL/BW ratio: 2), outline oval, moderately elevated, semi-carinated, slightly protruding apices; girdle wide, spiculose. Tegmentum colouration not uniform, mottled and very variable (Fig. 1A–B).

Head valve slightly wider than long, almost semicircular, five rays barely raised, apex clearly visible (Fig. 2A). Intermediate valves ellipsoidal, wide, anterior margin slightly convex at both sides of straight or little concave jugum; hardly raised striated jugal area (Fig. 1A–B, Fig. 2B–C). Tail valve roughly semicircular with central, slightly elevated mucro (Fig. 2E).

Tegmentum covered with densely packed to widely separated (density from 23 to 38 granules per 1 mm², mean = 31, n = 8 specimens) granules of variable shape, from oval to more or less elongate drop-shaped; granules raised, flat-topped, with one macroaesthete subcentral surrounded by 8–16 posteromedially located microaesthetes (Fig. 1A, Fig. 2A–C, E, Fig. 3B and Fig. 4A).

Articulamentum thick, apophyses large, broadly rectangular, rounded at anterolateral margins, separated by a wide sinus; insertion plates wide, continuous with apophyses, slits moderately deep (slit formula: 5/1/2) (Fig. 2).

Girdle wide, fleshy, usually brighter than tegmentum, deeply encroaching sutural areas. Highly variable colour of the tegmentum and perinotum; usually from creamy-white to olive-green and brown, with different degrees of blotches in which white, yellow and brown variously combined; in some specimens, colours can be very bright, for example, bright yellow and orange. (Fig. 1A–B). Perinotum covered with two kinds of spicules: evenly distributed, short, somewhat bent and distally striated spicules (up to 100 x 15 µm) and longer thick spicules (up to 260 x 40 µm), striated (some of them smooth), usually slightly curved, randomly interspersed amongst them (Fig. 3D). Perinotum ornamentation highly variable, observed variability in examined specimens: dorsal ornamentation just as described above or only short and densely arranged spicules or densely arranged, short but very thin, irregularly-curved spicules, with long and thick spicules interspersed amongst them. Sutural tufts with around 20–46 thick, smooth needles (up to 340 x 20 µm) intermingled with much shorter and thinner needles (Fig. 3C). Marginal fringe with long and finely-ribbed spicules (some spicules may have deep grooves); hyponotum beset with slightly flattened, distally finely striated spicules, similar to the short dorsal spicules, up to 110 x 18 µm (Fig. 3E).

Radula central tooth elongated, almost straight at the top and slightly keeled near base; first lateral tooth wing-shaped and smaller than the central; second (major) lateral elongated, with accessory plate tricuspidate, outer denticle slightly shorter than the others; cusps pointed (Fig. 3A).

Gills merobranchial, 13–15 ctenidia on each side.

Acanthochitona crinita is well known for its high variability of morphological features. However, it can be distinguished from other NE Atlantic relatives on the basis of the below-mentioned morphological characters. A. crinita can be distinguished from A. discrepans (Brown, 1827), based on the number, size and arrangement of aesthetes on tegmental granules (one macroaesthete subcentral surrounded by larger, posteromedially-located 8–16 microaesthetes in A. crinita vs. 1–2 macroaesthetes: a single macroaesthete located in posterior third, second, if present, located centrally, surrounded by very small, posteriomedially located 26–40 microaesthetes in A. discrepans), dorsal spicules (longer, thicker spicules up to 100 x 15 µm in A. crinita vs. smaller, thinner spicules up to 68 x 7.5 µm in A. discrepans), tuft needles (longer spicules up to 340 x 20 µm, surrounded by a smaller number of very short and thin needles in A. crinita vs. longer spicules up to 1000 µm x 60 µm, surrounded by a large number of somewhat shorter and much thinner needles in A. discrepans). A. crinita differs from A. fascicularis (Linnaeus, 1767) by the shape of the intermediate valves (ellipsoidal in A. crinita vs. triangular in A. fascicularis), jugal area (hardly raised, not sharply separated from the latero-pleural areas in A. crinita vs. raised, sharply separated from the latero-pleural areas in A. fascicularis), shape of dorsal granules (moderately to widely apart, oval to elongated oval in A. crinita vs. small, round, densely packed granules with the incision in the middle in A. fascicularis), density of tegmental granules (23–38 granules per 1 mm², mean = 31 in A. crinita vs. 42–70 granules per 1 mm², mean = 52 in A. fascicularis), number of microaesthetes (8–16 microaesthetes in A. crinita vs. 0–6 microaesthetes in A. fascicularis), number of bristles in the sutural tufts (20–46 in A. crinita vs. 55-120 in A. fascicularis). It differs from A. oblonga Leloup, 1981 by the shape of dorsal granules (moderately apart, oval to elongated oval in A. crinita vs. widely apart and very much elongated in A. oblonga), number and arrangement of microaesthetes (8–16 microaesthetes located posteromedially in A. crinita vs. 6–9 microaesthetes, located mainly in the central area in A. oblonga). It can be easily distinguished from A. pilosa Schmidt-Petersen, Schwabe et Haszprunar, 2015 by the shape of the IV valve (valve with the hind margin concave at both sides of the pronounced apex in A. crinita vs. valve with triangular posterior margin with no apex in A. pilosa), density of tegmental granules (moderately to widely apart in A. crinita vs. densely packed in A. pilosa), size of microaesthetes on the tegmental granules (bigger microaesthetes in A. crinita vs. smaller, ca. ½ of the diameter, in A. pilosa), large dorsal spicules (not always present, sparse, curved spicules of size up to 260 x 40 µm in A. crinita vs. always present, dense and straight spicules of size up to 320 × 60-62 µm in A. pilosa).

Acanthochitona crinita is a widely distributed species in North-East Atlantic. The species inhabits the seas from of north of Scotland (locus typicus), south of England (molecular data and SEM photos from this study), north of France and Spain, west coast of Portugal (molecular data from GenBank), the Azores (molecular data and SEM photos from this study) and, most likely, the Mediterranean Sea (literature data).

Chiton discrepans: Brown, 1827 - Brown 1827, pl. 35 fig. 20; 1844 - Brown 1844: 65, p. 21 fig. 20.

non Chiton discrepans: Sowerby II, 1840a: 2 [in synonymy of C. crinitus (non Pennant)] et mult. auct.

Chiton fascicularis: Brown, 1827 - Brown 1827 (ex parte): pl. 35 fig. 5; 1844 - Brown 1844: 65 (ex parte), pl. 21 fig. 5.

non Chiton fascicularis: Linnaeus, 1767 - Linnaeus 1767.

Chiton gracilis: Jeffreys, 1859 - Jeffreys 1859: 106, pl. 3 figs 9a-c; Sowerby II, 1859 - Sowerby II 1859: pl. 10 fig. 6; Winckworth, 1926 - Winckworth 1926: 15, pl. 1, fig. 1, lb-d.

Chiton fascicularis var. gracilis: Jeffreys, 1865 - Jeffreys 1865: 212; Dean, 1926 - Dean 1926: 21; Pilsbry, 1893 - Pilsbry 1893: 11, pl. 4 fig. 83.

Acanthochitona discrepans: Winckworth, 1926 - Winckworth 1926: 15, pl. 1 fig. 2; Dean, 1926 - Dean 1926: 21; Winckworth, 1932 - Winckworth 1932: 218.

Chiton gracile: Warén, 1980 - Warén 1980: 13.

Acanthochtona crinita: Kaas, 1985 - Kaas 1985: 591, Figs. 28-38.

Acanthochitona discrepans: Kaas, 1985 - Kaas 1985: 598, Figs. 59-75.

Type material. Lectotype: TENBM 1983.4588/1, Tenby Museum, Tenby, Pembroke, Wales. Designated by Kaas (1985) (material not seen).

Type locality. Wales, Pembroke, Tenby (Pennant 1777, Kaas 1985).

Animal small to medium size (in examined material, BL: 11–22 mm, BW: 5–11 mm, BL/BW ratio: 2.1), outline oval, moderately elevated (elevation ratio 0.41), semi-carinated, side slopes flat to slightly raised, apices not prominent; girdle wide, spiculose (Fig. 1C). Dorsal colouration not uniform, often mottled, variable; tegmentum brownish to dark red, lateropleural and antemucronal areas with blue, dark brown or beige maculation, jugum usually dark brown to black; colour of perinotum beige to pale yellow mottled with olive green or brown; normally 18 sutural tufts (Fig. 1C).

Head valve slightly wider than long, almost semicircular, anterior slope slightly convex, posteriormost margin straight without a notch (Fig. 1C, Fig. 5A). Intermediate valves ellipsoidal, wide, anterior margins slightly rounded at both sides of straight or little concave jugum, side margins rounded, very slightly beaked with hind margin slightly concave at both sides of the apex; smoothly rounded or semi-carinate; jugal areas striated, wide, wedge-shaped, little elevated and hardly separated from the lateropleural areas (Fig. 1C, Fig. 5B, C). Tail valve roughly semicircular with central, moderately-elevated mucro and posterior slope steep and straight (Fig. 5D).

Tegmentum uniformly covered with rather densely distributed (density from 24 to 50 granules per 1 mm², mean = 37, n = 8 specimens), oval to elongated granules, arranged in quincunx order, except for the jugal area of intermediate valves; the granules raised, flat topped, to slightly concave (Fig. 1C, Fig. 4B, Fig. 5, Fig. 6B). Proximal granules more elongate and merging into jugum; in lateral areas, granules spaces less than granule-width apart. Each granule with 1–2 macroaesthetes (single macroaesthete located in posterior third; second, if present, located centrally) surrounded by 26–40 very small posteriomedially located microaesthetes (Fig. 4B, Fig. 6B).

Articulamentum well developed, solid, slightly pinkish ventrally beneath tegmentum. Apophyses large, separated at jugum, broadly rectangular to wing-shaped, trapezoidal in tail valve (direct dorsal view can create appearance of triangular outline); insertion plates wide, continuous with apophyses, slits deep, extending proximally into shallow dorsal channels (slit formula: 5/1/2) (Fig. 5).

Girdle wide, fleshy, leathery looking, brighter than tegmentum, deeply encroaching sutural areas. Colour from beige or light yellow to pale brown with brown and olive-green streaks and blotches, in some specimens prominent dark green bands with fuzzy boundaries located near valves between the sutural tufts; sutural tufts with translucent or brownish spicules; base of sutural tufts creamy-yellow or brown (Fig. 1C). Dorsal surface densely covered by minute, thin brownish spicules (up to 68 x 7.5 µm) sculptured with fine riblets, pointed, but often broken (Fig. 6D, F). In some specimens, randomly interspersed longer and thicker spicules, located only in outer half of the girdle identical to spicules of marginal fringe (Fig. 6F). Sutural tufts prominent, with around 15–40 thick, sharply pointed, smooth needles, measuring up to 1000 µm x 60 µm, surrounded by much shorter and thinner, but also smooth bristles (Fig. 6C). Marginal fringe with rounded, solid spicules, ribbed on proximal half, distal half ribbed or smooth, up to 300 x 35 µm. (Fig. 6E–F). Hyponotum densely covered with imbricating flattened, elongated spicules, finely striated near the top with 3-4 ribs, much larger than dorsal spicules, up to 120 x 20 µm (Fig. 6F).

Radula central tooth elongated, without cusp, almost straight at the top, keeled near base; first lateral tooth wing-shaped and wrapping around central; second (major) lateral massive with accessory plate tricuspidate with outer denticle shorter than the others; cusps pointed and triangular in outline (Fig. 6A).

Gills merobranchial, composed of 9–13 ctenidia on each side reaching around half body length.

This species can be distinguished from other Acanthochitona species from NE Atlantic on the basis of a set of morphological characters. It differs from A. crinita (Pennant, 1777) by the number, size and arrangement of aesthetes on tegmental granules (1–2 macroaesthetes - a single macroaesthete located in posterior third, second, if present, located centrally, surrounded by very small, posteriomedially located 26–40 microaesthetes in A. discrepans vs. one macroaesthete subcentral surrounded by larger, posteromedially located 8–16 microaesthetes in A. crinita), dorsal spicules (smaller, thinner spicules up to 68 x 7.5 µm in A. discrepans vs. longer, thicker spicules up to 100 x 15 µm in A. crinita), tuft needles (longer spicules up to 1000 µm x 60 µm, surrounded by a large number of somewhat shorter and much thinner needles in A. discrepans vs. longer spicules up to 340 x 20 µm, surrounded by a smaller number of very short and thin needles in A. crinita). Acanthochitona discrepans can be distinguished from A. fascicularis (Linnaeus, 1767) by the shape of the intermediate valves (ellipsoidal in A. discrepans vs. triangular in A. fascicularis), jugal area (hardly raised, not sharply separated from the latero-pleural areas in A. discrepans vs. raised, sharply separated from the latero-pleural areas in A. fascicularis), shape of dorsal granules (decidedly apart, oval to elongated oval in A. discrepans vs small, round, densely packed granules with the incision in the middle in A. fascicularis), density of tegmental granules (24–50 granules per 1 mm², mean = 37 in A. discrepans vs. 42–70 granules per 1 mm², mean = 52 in A. fascicularis), number of microaesthetes (26–40 microaesthetes in A. discrepans vs. 0–6 microaesthetes in A. fascicularis), number of bristles in the sutural tufts (15–40 in A. discrepans vs. 55-120 in A. fascicularis). Acanthochitona discrepans differs from A. oblonga Leloup, 1981 by the shape of dorsal granules (rather densely distributed, oval to elongated granules, arranged in quincunx order in A. discrepans vs. widely apart and very much elongated in A. oblonga), number and arrangement of microaesthetes (26–40 microaesthetes located posteromedially in A. discrepans vs. 6–9 microaesthetes located mainly in the central area in A. oblonga). It can be distinguished from A. pilosa Schmidt-Petersen, Schwabe et Haszprunar, 2015 on the basis of the shape of the IV valve (very slightly-beaked valve with hind margin slightly concave at both sides of the apex in A. discrepans vs. valve with triangular posterior margin with no apex in A. pilosa), number of microaesthetes on the tegmental granules (26–40 microaesthetes in A. discrepans vs. around 14 microaesthetes in A. pilosa), large dorsal spicules (only sometimes present and very sparse large dorsal spicules in A. discrepans vs. always present, dense and straight spicules in A. pilosa).

This species seems to have a more northern distribution than Acanthochitona crinita. The range of species is from Norway (molecular data and SEM photos from this study), through the north coast of Ireland (molecular data and SEM photos from this study) to the south of Wales (locus typicus). The literature records from more southern regions must be treated with caution as they probably represent other misidentified Acanthochitona species.

The original locus typicus of Acanthochitona discrepans (Brown, 1827) is in Tenby, Pembroke in Wales. Kaas (1985) chose the lectotype from the syntype series (Lyons collection, Tenby Museum). In our study, we re-describe this species, based on the specimens from the Strangford Lough from the Northern Ireland (ca. 320 km north from the locus typicus). However, our specimens in all aspects correspond to the description provided by Kaas (1985); Kaas also examined the specimens from Strangford Lough, confirmed that they are A. discrepans and stated that this species is fairly common there.

Chiton fascicularis: Linnaeus, 1767 - Linnaeus 1767: 1106; Poli, 1791 - Poli 1791: 10, pl. 4 fig. 3; Reeve, 1847 - Reeve 1827: pl. 10 sp. & fig. 53.

non C. fascicularis: Sowerby, 1840a - Sowerby II 1840a: figs. 87, 87a, et mult. auct.

Acanthochites communis: Risso, 1826 - Risso 1826: 269.

Chiton fascicularis var. major: Philippi, 1836 - Philippi 1836: 108, pl. 7 fig. 2a, b.

Chiton crinitus: Sowerby, 1840: figs. 88-93; 1840a: 2.

non C. crinitus: Pennant, 1777 - Pennant 1777.

Chiton discrepans: Sowerby, 1840a: 2 (in synonymy of C. crinitus; Sowerby, non Pennant, 1777); Forbes & Hanley, 1849 - Forbes and Hanley 1849: 396, pl. 58 fig. 4; Jeffreys, 1859 - Jeffreys 1859: 106, pl. 3 fig. 10, Jeffreys, 1869 - Jeffreys 1869: 198, pl. 5. fig. 4, et mult, auct.

non C. discrepans: Brown, 1827 - Brown 1827.

Acanthochites carinatus: H. Adams & Angas, 1864 Adams and Angas 1864.

non Acanthochites carinatus: Risso, 1826 - Risso 1826.

Chiton fascicularis var. rubra: Issel, 1870 - Issel 1870: 4.

Acanthochites discrepans var. minorflava: di Monterosato, 1878a - di Monterosato 1878: 78.

Acanthochites hamatus: de Rochebrune, 1882 - de Rochebrune 1882: 191; Thiele, 1909 - Thiele 1909: 43.

Anisochiton discrepans vars elongata, marmorata, nigrolineata: Dautzenberg, 1893 - Dautzenberg 1893: 25.

Acanthochites discrepans var. albina: Dautzenberg & Durouchoux, 1900 - Dautzenberg and Durouchoux 1900: 15.

Anisochiton (Acanthochites) discrepans var. viridis: Pallary, 1902 - Pallary 1902: 28.

Acanthochites discrepans var. violaceolimbata: Dautzenberg & Durouchoux, 1906 - Dautzenberg and Durouchoux 1906: 15.

Acanthochiton(a) communis: Winckworth, 1926 - Winckworth 1926: 15, et mult. auct.

Acanthochiton heterochaetus: Bergenhayn, 1931 - Bergenhayn 1931: 20, pl. 1 figs. 38-42, pl. 3 figs 67-74.

Acanthochiton discrepans var. angustivalvus: Bergenhayn, 1931 - Bergenhayn 1931: 20.

Acanthochiton communis f. barashi: Leloup, 1969 - Leloup 1969: 1, figs. 1, 2D, 3D, G, 4B.

Acanthochitona bonairensis: Kaas, 1972 - Kaas 1972: 44, text figs 72-73, pl. 3 figs. 1, 2; Watters, 1981 - Watters 1981: 173 (in synonymy of A. communis).

Acanthochitona fascicularis: Kaas, 1985 - Kaas 1985: 585, figs. 1-6.

Type material. Neotype: MNHN-IM-2000-5923, Muséum National d’Histoire Naturelle, Paris, France. Designated by Kaas (1985) (material not seen).

Type locality. Algeria, coast of Barbary (Linnaeus 1767); Algeria, Oran (Kaas 1985).

(See Jones and Baxter (1987), Dell’Angelo and Smriglio (2001), Bonfitto et al. (2011), Schmidt-Petersen et al. (2015) for additional descriptions that pertain to Acanthochitona fascicularis). Small to large size (in examined material, BL: 9–30 mm, BW: 5–18 mm, BL/BW ratio: 1.7), outline oval, flat to moderately elevated, semi-carinated, slightly protruding apices; girdle very wide and spiculose. Dorsal colouration variable; usually green and brown variously combined; some specimens can be uniform and brightly coloured, yellow or orange (Fig. 1D–E).

Head valve much wider than long, semicircular, with five barely-raised rays, posterior margin almost straight, no notch and apex (Fig. 7A). Intermediate valves from triangular to trapezoid, apex slightly protruding; raised, clearly separated from lateropleural areas, wedge-shaped and striated jugum (Fig. 1D–E, Fig. 7B). Tail valve almost circular with central elevated mucro (Fig. 1D, Fig. 6C).

Tegmentum uniformly and thickly covered (density from 42 to 70 granules per 1 mm², mean = 52, n = 7 specimens) by small, round granules arranged in arched lines on the valves; each granule with 0–6 aesthetes randomly distributed, without a clear separation between macro- and microaesthetes; some granules with an incision in the middle of the granule posterior margin (Fig. 1D, Fig. 4C, Fig. 7, Fig. 8B).

Articulamentum well-developed; apophyses wide, rectangular, separated at jugum, rounded at anterolateral margins; insertion plates wide, continuous with apophyses, slits deep, extending dorsally into shallow channels (slit formula: 5/1/2) (Fig. 7).

Girdle very wide and fleshy, deeply encroaching sutural areas. Colours ranging from creamy-yellow to olive-green and brown, with different degrees of blotches in which white, green and brown variously combined; sometimes yellow or orange (Fig. 1D–E). Dorsal perinotum densely covered with two kinds of spicules: shorter, slender, pointed, but usually broken spicules and longer and thicker spicules interspersed amongst them (Fig. 8D). Sutural tufts with 55–120 long and slim bristles, surrounded by shorter, thinner, also smooth needles. Hyponotum densely covered with lanceolate, flattened spicules, finely striated near the top, of size up to 100 x 18 µm (Fig. 8C).

Central radula teeth elongated, almost straight at the top and keeled near base; first lateral tooth wing-shaped and smaller than the central; second (major) lateral elongated with accessory plate tricuspidate with central denticle longer than the others; cusps pointed and triangular in outline (Fig. 8A).

Gills merobranchial, 15-17 ctenidia on each side.

Acanthochtiona fascicularis, can be easily distinguished from other NE Atlantic relatives by the triangular shape of valves, raised and clearly separated jugal area, small and round, densely-packed tegmental granules and small number of aesthetes (0–6) without clear separation between micro- and macroaesthetes. For comparison, see the Remarks section for A. crinita and A. discrepans.

This species is widely distributed in the North-East Atlantic, with a range from the west coast of Ireland (molecular data and SEM photos from this study), south to the Azores (molecular data and SEM photos from this study) and the Mediterranean Sea (literature records and GenBank sequences).