TELOPaTA — CYREXA. 11 



k'sil impressiou ending in a very short sinus ; ligament short, 

 narrow. 



Tills genus was established by Mr. Gray for a shell which he 

 had previously described under the name of Cijrena ; it differs very 

 materially from the other genera of the family in its exterior shape 

 and in the peculiar formation of the hinge. The valves are very 

 thick, oblong, the beaks heart-shaped, and the lunula is very dis- 

 tinct and convex in the centre. The hinge is broad, it has three 

 cardinal teeth, all inclined obliquely towards the posterior side, 

 the anterior lateral tooth is very thick, prominent, and is placed 

 very close to and at a right angle with the anterior cardinal tooth, 

 the posterior lateral tooth is long, somewhat narrow, and to a cer- 

 tain extent similar to that of Corhicula. 



We know of but one species of Velorita, a recent one from 

 Japan, the V. cyprinoides. 



Mr. Deshayes, who does not admit the validity of this genus, 

 has discovered in the Basin of Paris certain forms of Cyrena, 

 which would seem to establish a connecting link between Velorita 

 and Cyrena. 



(This genus is not represented on this continent.) 



CYREXA, Lamarck.' 



Venus, Chemn. 1769.— Cyc/as, Becg. 1792.— Cyrena, Lam. 1818.— 

 Cyanocyclas, Feb. 1818. — Polymesoda, Raf. 1820. — Mnctra, 

 Bkongt. 1823.— Geloina, Gray, 1844. 



Animal oval, transverse ; mantle lobes free on the lower edge 

 and in front, united behind into two short syphons ; foot large, 

 compressed, ovate, trigonal ; tentacles elongate, ovate, trigonal ; 

 gills unequal, the internal ones subquadrangular, the external ones 

 smaller, subtrigonah 



Shell oval or subtrigonal, thick, solid ; three cardinal teeth in 

 each valve somewhat divergent ; two lateral teeth, unequal, the 

 anterior one situated nearer the cardinal teeth ; palleal impression 

 variable. 



The genus Cyrena, as originally constituted by Lamarck, was 

 ' Lamarck, Anim. s. vert., V, 1818, 551. 



