PETRICOLA. 151 



can be perceived with a lens of the highest power ; there is, 

 however, a distinct callus beneath the ligament. 



PETRICOLA, Lamarck. 



Shell ovate or subtrigonal, ventricose, equivalve, in- 

 equilateral, gaping : hinge with two cardinal teeth in each 

 valve, or two in one, and one in the other valve. Liga- 

 ment external short. Pallial impression, with an ample 

 and rounded sinus. 



Animal oval. Mantle closed in front except a small 

 opening for a lanceolate and pointed foot. Siphons united 

 for nearly half their lengths ; their orifices fringed with a 

 double series of cirrhi, the longer ones pinnated on one 

 side. 



Notwithstanding that the Petricola, from the compara- 

 tively impregnable nature of the fortifications in which 

 they entrench themselves, are justly esteemed among the 

 less frequent sojourners in our cabinets, the genus has 

 a very wide distribution, pervading the temperate and 

 southern parts of Europe, the Red Sea, the islands of 

 the S. Pacific, Australia, and the shores of South- West- 

 ern and North America. 



P. LITHOPHAGA, RctziuS. 

 Plate VI. figs. 9, 10, and (animal) Plate G. f. I. 



Venus lithopkaga ,Retzius, Trans. Turin. 1786. 

 ( Unnamed.) PoLi, Test. Sicil. vol. i. pi. 7, f. 14, 15. 



M^a decussata, Mont. Test. Brit. Suppl. p. 20, pi. 28. f. 1. — Turt. Conch. 

 Diction, p. 102. — Flem. Brit. Anim, p. 466. — Brit. Marine 

 Conch, p. 41. — Wood, General Conch, p. 99. — Dillw. Re- 

 cent Shells, vol. i. p. 46.— Index Test. pi. 2, Mya, f. 1 7. 

 . Petricola striata. Lam. Anim. s. Vert. ed. 2, vol. vi. p. 150. — Deles. Rcc. 

 Coquil. pi. 4, f. 11. — Hanl. Recent Shells, p. 52, suppl. pi. 

 ll,f. 44. 



