516 CRENELLIKffl. 



Shell oblong, equivalve, inequilateral; surface of valves 

 smooth, concentrically striated, or radiately sulcated, in- 

 vested with an epidermis often filamentose ; anterior side 

 very short; beaks nearly lateral, depressed on the short 

 side. Hinge-margin toothless, or with only a compressed 

 tooth-like callus, and rarely corrugated; ligament linear, 

 almost internal, placed in a marginal canal. Muscular 

 scars very unequal; pallial impression obscure. 



Sijn. Volsella, Scopoli. Pholas, Klein, not Linn. Ta- 

 rn arindiforrnis (b), Gevers. Modiola, Lam. Modiolus, 

 Risso. 



Ex. P. Americana, D'Herbigny, pi. 121, fig. 4. Shell, 

 P. modiolus, Linnmis, fig. 4, a, 4, b. 



The Pernce or Modiohe are distinguished from the 

 Mytili by their habit of burrowing or spinning a sort of 

 woven nest of stones and fragment of shells, in which they 

 occasionally conceal themselves. There are upwards of 

 seventy species, chiefly tropical in their geographical dis- 

 tribution, though a few have been observed in Britain, and 

 in the Mediterranean and Arctic seas. Numerous fossil 

 species are obtained from the Lias in Europe, India, and 

 the United States. 



Species of Perna. 



aberrans, C. B. Adams. areola, Gould. 



abyssicola, C. B. Adams. auriculata, Erauss. 



Adriatica, Lam. australis, Gray. 



albicosta, Lam. Ballii, Brown. 



Americana, D'Herb. barbata, Linn. 



Antillarum, Phil. biradiata, Haul. 



arata, Dkr. Brasilieusis, Chem. 



arborescens, Chem. Californiauus, Eschsch. 



arcuatula, Hani. capax, Conr. 



