86 



MOLLUSCA. 



[Cardiacea. 



1. Arca No*, pi. XXXIII, f. 1, 2, 3. 



Area Noce, First Ed., pi. 25, f. 1, 2, 3; Linne, Syst. Nat., 

 p. 1140, No. 169; Lamarck, VI, pt. 1st, p. 37; Montagu, p. 

 139, pi. 4, f. 3, 3 ; Donovan, V, pi. 158, f. 1,2; Turton, Biv., 

 p. 166; Fleming, p. 397- 



Shell rliomboidal, cymbiform ; the smaller end of each valve 

 turning inwards to meet each other ; umbones situate nearest 

 to one side, separated by a pretty wide cardinal area, which is 

 smooth and glossy ; apex prominent, slightly inflected ; hinge 

 parallel, with numerous teeth, those in the centre perpendicular, 

 and oblique towards the sides; external surface with numerous, 

 longitudinal, divergent stria,-, which emanate at the apex, and 

 terminate on the margins, those on the narrower side wider 

 and stronger, assuming nearly the aspect of ribs, the whole 

 crossed by fine, transverse stria;, producing an elegant reti- 

 culated appearance ; colour rufous-brown ; inside white, or 

 slightly tinged with purplish-brown, with the margins slightly 

 crenated. 



Found on Milton Sands, south coast of Devon ; at Land's 

 End, Cornwall ; and also at Guernsey. 



2. Arca fusca, pi. XXXIII, f. 4, 5. 



Arcafusca, First Ed., pi. 25, f. 4, 5 ; Lamarck, VI, pt. 1st, 

 p. 39; Donovan, V, pi. 158, f. 3, 4 ; Montagu, Sup., p. 51 ; 

 Turton, Biv., p. 167 ; Fleming, p. 397. 



Shell cymbiform, transversely elongated; umbones prominent, 

 rounded, obtuse, and somewhat remote, being separated by a 

 pretty broad, flat, cardinal area, and situated nearer one end ; 

 whole outer surface fuscous-brown, covered with fine, divergent, 

 longitudinal striae, which are decussated by transverse, sharp 

 stria; ; posterior side somewhat truncated ; inside glossy, and 

 white ; margin with very fine crenulations. 



Found at Cornwall and south coast of Devon. Lady Jardine 

 possesses a specimen which was found near Weymouth. 



This shell differs from A. Noce in being longer in proportion 

 to its breadth, in the striae being finer, and the colour more of 

 a yellowish-brown. 



3. Arca lactea, pi. XXXIII, f. 6. 



Arca lactea, First Ed., pi. 25, f. 6; Lamarck, VI, pt. 1st, p. 

 40; Montagu, p. 138; Donovan, IV, pi. 135; Fleming, p. 398; 

 Arca perforans, Turton, Biv., p. 169, pi. 13, f. 23. 



Shell subrhomboidal, rounded at both extremities ; umbones 

 not quite central, blunt, and placed somewhat remote ; cardinal 

 area smooth ; hinge parallel, with numerous teeth ; outer surface 

 covered with a pilous, pale yellowish-brown epidermis, beneath 

 which the shell is milk-white, with numerous, close-set, regular, 

 longitudinal striae, the interstices between the stria; finely punc- 

 tured, producing the appearance of being reticulated, when 

 viewed through a lens, crossed by a few concentric wrinkles, or 

 lines of growth; inside white, with a plain margin. Length half 

 an inch ; breadth three-quarters. 



The sides in the young shells are for the most part oblique, 

 and subtruncated, and assume a more rounded form as the shell 

 advances in growth; some specimens have also transverse striae, 

 which, however, is but of rare occurrence. 



Found plentifully on the Devonshire and Cornwall coasts. 



4. Arca barbata, pi. XXXIII, f. 7- 



Arca barbata, First Ed., pi. 25, f. 7 ; Brown, Wernerian 

 Mem., II, p. 512, pi. 24, f. 3; Lamarck, VI, pt. 1st, p. 59; 

 Fleming, p. 398; Area reticulata, Turton, Biv., p. 168. 



Shell transversely elongated, strong, white, very finely reti- 

 culated ; covered with a reddish-brown epidermis ; from the 

 umbones to the margin diverge a number of nearly equidistant, 

 flat ridges, covered with very fine, short, and thick-set brown 

 hairs ; all round the margin it is beset with a byssus of rather 

 long, stiff bristle-like hairs ; in the hollows at the anterior and 

 posterior sides, it is thickly covered with bristles of a very dark 

 umber-brown, rather small at the umbo, and increase in size as 

 they diverge from it on both sides, and oblique in opposite 

 directions ; inside moderately glossy, growing dimmer as it 

 approaches the umbones of the shell, white, with pale glossy 

 rays, and clouded with pale chestnut ; margin finely crenulated. 

 Breadth nearly double its length. 



Discovered in Lough Strangford, County of Down, Ireland, 

 by Dr. Macgee, of Belfast. 



5. Arca tetragona, pi. XXXIII, f. 20, 21. 



Arca tetragona, Forbes, Mai. Mon., p. 41 ; Poli, p. XXV, 

 pi. 25, f. 13? 



Shell transversely oblong, obliquely quadrangular, very tu- 

 mid ; an oblique rib takes it rise at the umbo, and terminates 

 on the anterior side ; umbones but slightly produced, and very 

 remote, separated by a very large, flat, nearly lozenge, cardinal 

 area, from the sinistral side of which it suddenly sinks, forming 

 an acute angle, by the meeting of the arcuated basal line ; car- 

 dinal area with a large lozenge impression, subquadrangularly 

 striate, the lines of the one valve meeting those of the other; 

 whole surface with longitudinal and transverse stria;; colour 

 brownish-white ; covered with a thin, pilous epidermis, and 

 fringed round the margins of the valves, which are longer at 

 the beak ; basal margin provided with a large, central, oblong, 

 lozenge hiatus, for the passage of the byssus ; inside white, 

 tinged with purple at one side ; hinge line straight, with many- 

 teeth, which are numerous posteally and few anteally, the mar- 

 gin with fine crenulations. 



Found in the Calves on the West coast of Ireland, where it 

 burrows in hard clay and limestone rocks ; and dredged by 

 Professor Forbes off the coast of Ballaugh, Isle of Man. I 

 obtained it from Mr. Richardson, with one of the specimens, 

 embedded in limestone. 



Family III Cardiacea. 



Primary teeth irregular both in form and situation, and in 

 general accompanied by one or two lateral teeth. 



Genus 7. — Isocardia. — Lamarck. 

 Shell equivalve, heart-shaped, ventricose; beaks very distant, 

 divergent, and involute; hinge with two primary compressed 

 teeth in each valve, the one next the apex inflected under the 

 umbo, and with one elongated, lateral tooth, situate imme- 

 diately before the ligament, which is external, and divided into 

 two ligaments at its posterior extremity, both of which are 

 divergent to the point of the beak in each valve ; both valves 

 provided with two lateral, ramote, muscular impressions, the 

 linear impression of the mantle is entire, and extending from 

 one muscular impression to the other. 



1. Isocardia Cor, pi. XXX, f. 9, and pi. XXX,* f. 5. 

 Isocardia Cor, First Ed., pi. 23, 24 ; Lamarck, VI, pt. 1st, 

 p. 31 ; Turton, Biv., p. 193, pi. 14; Fleming, p. 419; Chama 



