348 INTRODUCTION TO CONCHOLOGY. 



Gnathodon has a shell that is nearly oval^ equivalve, and 

 inequilateral, covered with an olive-green epidermis, and 

 generally a little decorticated at the umboes. A lateral and 

 two cardinal teeth are apparent in one valve, shutting 

 between two cardinal and two lateral teeth in the other. 

 The ligament is internal, inserted in a deep pit between the 

 cardinal and posterior lateral teeth, and the impression of 

 the mantle exhibits a small sinus. 



Shells of the genus Crassatella exhibit an interesting 

 peculiarity of character: they are both thick and solid, with 

 an internal ligament, and rich dark brown epidermis. 

 Eleven species were referred to tliis genus by Lamarck, but 

 six only are allowed to remain. Since then, three new 

 species have been described by Sowerby, and ten by Mr. 

 Reeve, in the Proceedings of the Zoological Society. 



Several specimens of the Swollen Crassatella {C. gihhosa) 

 were dredged up by Mr. Cuming from sandy mud in eleven 

 fathoms water, off St. Elena, and Xipixpi, in South America. 

 Those figured in the ' Conchologia Iconica ' are the oldest 

 and the youngest of these; they are admirably adapted 

 to show the different periods of growth : — when young, the 

 valves are depressed and strongly undulated, after which 

 they gradually assume a gibbous form, and the undula- 



