80 MACTRAD^. 



the same size. The raised hues are, however, thin, elevated, sharp 

 ill this, while in C. contracta they are thick and rounded, and the 

 shell is so much more compressed as to preclude mistake. 



A species of this genus is found in the West Indies, perhaps the 

 same, and one or two more have l^een found in the Pacific ; and 

 these are all the species at present known. 



From an examination of the animal, Dr. 8timpson has trans- 

 ferred this genus to the family Tellinidce. The siphons are quite 

 separate, and the warped posterior part would justify this change. 



Genus CEBONIA, Gray. 1849, 



Shell oval, wedge-shaped, truncated posteriorly ; lateral teeth 

 sub-equal, compressed, furrowed. Siphonal sinus distinct. 



Ceronia arctata. 



Fig. 39. 



Shell sub-triangular, very inequilateral, truncated posteriorly, smooth and cov- 

 ered by a shining yellow epidermis ; lateral teeth straight, striated. 



Mactra arctata, Conrad, Journ. Ac. Nat. So. vi. 257, pi. 11, fig. 1 (1830). 



Mactra deaurata, Conrad, Amer. Mar. Conch. 59, pi. 14, fig. 1. 



Mactra subtriangiilata, WooD, Index, Supjil. pi. 1, fig. 10. — Griffith's, Cuv. (Mollusca) 



pi. 22, fig 4. 

 Mesodesma arctata, Gould, Invert. 1st eel. 57. — Stimpson, Shells of New England, 20. 



-De Kay, New York Moll. 231, pi. 23, fig. 288. 

 Ceronia arctata, Ciienu, Man. dc Couch, ii. 79, fig. 340. 



Shell sub-triangular, wedge-shaped, thick and strong, very in- 

 equipartite, the posterior part very short, forming the base of the 

 wedge, its lower point truncated ; upper margin straight ; ante- 

 rior part narrowed, regularly rounded, as 

 Fig. 891^^^^^ jg g^lgQ ^YiQ ventral margin ; beaks very 



slightly elevated, erect ; a sharp ridge 

 passes from them to the hinder and lower 

 angle ; surface with occasional rounded 

 ridges at the stages of growth, covered 

 with a thin, golden-yellow epidermis re- 

 flecting a metallic lustre : hinge com- 



C. arctata. ~ 70 



posed of a very deep, spoon-shaped cav- 

 ity for the cartilage ; a long V tooth opening at a very acute 

 angle, and on each side a straight lateral tooth, partially double in 

 the riglit valve, their articulating surfaces striated; posterior tooth 



