366 INTllODUCTION TO COXCHOLOGY. 



of their shells, together with the habits of the animal, being 

 so peculiar and distinct as not to admit of further sub- 

 division. The shell is equivalve, transversely oblong, very 

 inequilateral, 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 curva- 

 ture or tooth. The external surface is uniformly of a 

 delicate white colour, generally crossed longitudinally and 

 transversely with fine muricated striae. The interior is 

 colourless, or rather of a pure or dusky white, though it 

 sometimes partakes of a brownish cast; this deficiency is, 

 however, amply compensated by the beautiful fretwork with 

 which the shells of this genus are usually embellished. The 

 accessory pieces are irregular in number, equally with their 

 places of attachment, being formed within or on any part 

 that requires protection, according to the necessities of the 

 animal. They are, consequently, found more generally over 

 the hinge. The animal resembles a fleshy membranaceous 

 bag, nearly the length of the shell, and open at each end ; 

 furnished at the upper part with a cylindrical muscular 

 tube, divided by a partition ; at the lower, with a short, 

 obtusely conical foot. 



