92 CONCHIFERA. DIMYAIRA. 



incurved, and the margin crenated ; colour rusty brown, mottled 

 or striped with white, chestnut, orange, or yellowish. 2 inches 



long. Inhabits Mediterranean and Atlantic seas. B Brown's 



Illust. pi. 25, fig. 8, 9. 



P. pilosus, Lam. Shell orbicular, oval, tumid, with decussated 

 striae ; umbones oblique ; epidermis fuscous, pilose ; a large red- 

 dish-brown spot in the interior. 2 inches long. Inhabits Medi- 

 terranean sea. B Brown's Illust. pi. 25, fig. 10, 11. 



** With projecting radiated longitudinal ribs. 



P. pectiniformis, Lam. (A. pectunculus, Lin.) Shell suborbicular, 

 thick, slightly eared, white, spotted with reddish-brown; ribs 

 thick, transversely striated ; umbones small ; margin plaited. 1^ 

 inch in diameter. Inhabits Indian and American seas. Lister, 

 pi. 239, fig. 73. 

 The fossil species of this genus are numerous, and have been found in Britain 



in the oolite, green-sand, and London clay. 



Gen. 12. ARC A, Lam. Lin. 



Shell transverse, subequivalve, inequilateral, with the beaks se- 

 parated by the ligament ; hinge in a straight line, without 

 ribs at the extremities, and furnished with numerous teeth ; 

 ligament external. 



The shells of this genus are remarkable for the distance between their beaks. When 

 the shell is reversed or placed upon its upper margin, it presents somewhat the ap- 

 pearance of a ship, particularly the species which are elongated transversely, and 

 hence the generic name. The broad flat space between the apices is marked by fur- 

 rows, which form lozenges when the valves are united. The Arcto are all marine 

 shells, and many of them are covered with a scaly epidermis. 



* Upper margin not crenated. 



A. tortuosa, Lin. Shell parallelepiped, distorted, with the valves 

 and sides unequal, and obliquely keeled; beaks small, recurved; 

 colour reddish white. About an inch long, and thrice as broad. 

 Inhabits Indian seas. D'Argenv. pi. 19, fig. 1. 



A. Noce, Lin. Shell oblong-rhomboidal, strongly striated longitu- 

 dinally, with the apices incurved and very remote ; margin entire 

 and gaping ; colour whitish, with diagonal parallel zigzag chest- 

 nut stripes. 2 inches broad. Inhabits European seas. B. 

 Brown's Illust. pi. 25, fig. 1, 2, 3. 



A. barbata, Lin. Shell transversely oblong, rounded at both ends, 

 and marked .with decussated striae ; summits approximated ; mar- 

 gin entire and nearly closed ; epidermis reddish brown, bearded 

 with dark bristles. "l| inch long. Inhabits seas of Europe. B. 

 Brown's Illust. pi. 25, fig. 7- 



** Upper margin crenated within. 



A. antiquata, Lin. Shell obliquely heart-shaped, with rounded slight- 

 ly wrinkled longitudinal ribs ; apices remote and recurved ; mar- 

 gin crenated ; colour white, tinged with pale flesh colour at the 

 summit. 2 inches long and 3 broad. Inhabits Indian and Eu- 

 ropean seas Poli, Test. pi. 25, fig. 14, 15. 



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