40 CLASS I. TROPIOPODA. ORDER I. BIMUSCULOSA. 



two distinct muscular impressions, the posterior large, the anterior 

 smaller, situated near the edge of the superior margin. The Xylo- 

 phaga is generally found in light wood or pieces of stick, which it pene- 

 trates to the depth of about an inch. 



Example. 



PL XXII. Fig. 1 to 4. 



Xylophaga dorsalis, Turton. Sowerby, Genera of Shells, No. 29. 

 Pholas xylophaga, Deshayes. 

 Xylotrya dorsalis, Leach. 



PHOLAS, Linnaeus. 



Testa transversa, oblonga, aequivalvis, inrequilateralis, utroque latere 

 hians, hiatu antico plerumque maximo, interdum fere clauso, parti- 

 bus accessoriis testaceis difformibus supra vel infra ssepius instructa. 

 Cardo dente longo, curvo, in utraque valva infra marginem umbo- 

 nalem prominente. 



Tbe Pholades constitute one of the few genera established by Linna?us 

 that remain nearly entire ; the construction of their shells, together with 

 the habits of the animal, being so peculiar and distinct, as not to admit 

 of further subdivision. 



The shell of Pholas is equivalve, transversely oblong, very inequila- 

 teral, and gaping more or less at both ends ; the dorsal margin, being 

 very much reflected back, is generally divided by numerous transverse 

 septa, and from within the umbo of each valve proceeds a strong, 

 spoon-shaped curvature or tooth. The external surface of the shell is 

 uniformly of a delicate white colour, generally crossed longitudinally 

 and transversely with fine muricated stria;. Tbe accessory pieces are 

 irregular in number as well as in their place of attachment, being formed 





