152 GASTROCH^NIDTE. 



Petricohi costcllala. Lam. Anim. s. Vert. ed. 2, vol. vi. p. 1.58. — Deles. Rec. 



Coquil. pi. 4, f. 12. — Hanl. Recent Shells, p. 52, suppl. 



pi. 11, f.45. 

 „ rocceUaria, Lam. Anim. s. Vert. ed. 2, vol. vi. p. 158. — Desh. Elem. 



Conch, pi. 12, f. 7.— Deles. Rec. Coquil. pi. 4, f. 13.— 



Hanl. Recent Shells, p. 52, suppl. pi. 11, f. 46. 

 „ riiperella. Lam. Anim. s. Vert. ed. 2, vol. vi. p. 159. — Deles. Rec. 



Coquil. pi. 4, f. 14. — Hanl, Recent Shells, p. 52, suppl. 



pi. ll,f. 47. 

 V^n6rupe pSlricole, Blainv. Man. Malac. pi. 76, f. 2. 

 Petrkola litJwpIiaga, Philifpi, Moll. Sicil. vol. i. p. 21, pi. 3, f. 6, and vol. ii. p. 



20, — Phil, in Wiegmann Archiv. Naturg., 1845, p. 188, 



p. 7, f. 11 to 14 (animal). — Exped. Scient. Algerie, 



Moll. pi. 66, f. 5 to 9 (shell), and pi. 67 (anatomy). 

 SpJuenia decussata, Turt. Dithyra, Brit. p. 38. — Bkown, Illust. Conch. G. B. 



p. 104, pi. 45, f. 3. 

 PHricok costellee, Chenu, Traite Elem. p. 200, f. 59. 



The shape of this Petricola varies from obovate to 

 elongated ovate, and the solidity from rather thin to 

 moderately strong. It is typically very inequilateral, but 

 the inequality of its sides is often rendered less observable 

 by the impeded development of the hinder one : this 

 character, however, is almost always observable by noting 

 the direction of the earlier lines of increase. The valves 

 are ventricose, and sometimes even tumid in front, the 

 hinder portion being always less convex. They are of a 

 dull lustreless white, with, in one (the largest) specimen, 

 an internal purplish stain at the posterior extremity. The 

 surface is roughened by numerous radiating elevated lines, 

 Avliich, in the only two British examples we have seen, be- 

 come narrow ribs posteriorly ; and gradually diminish in 

 breadth, but with an increasing proximity to each other, 

 towards the anterior extremity, before arriving at which, 

 they become obsolete or very nearly so, A kind of de- 

 cussation is produced from these lines being traversed by 

 minute concentric wrinkles and striae of increase, but in 

 ordinary (Mediterranean) specimens this is by no means 



