236 NEW- YORK FAUNA — MOLLUSCA. 



GENUS COCHLODESMA. Couthouy. 



Animal with a thin mantle, closed by a membrane in front, except at the antero-inferior 

 extremity, where it gives passage to a broad compressed foot extending along the whole 

 inferior surface of the abdominal mass. Edges of the pallium thickened, and a little rugose. 

 Siphons long, narrow and divided in their whole extent, and opening separately into the 

 branchial cavity. Shell thin, fragile, inequivalve, inequilateral ; right valve most convex. 

 Beaks moderately prominent, cloven ; ligament double. Hinge a spoon-shaped process in 

 each valve, supported by one or more oblique ribs. Palleal impression deeply indented 

 behind. 



COCHLODESMA LEANA. 

 PLATE XXXI. FIGS. 299, 301. a. b. 



(STATE COLLECTION.) 



Ajtatina leana. Conrad, Jour. Acad. Nat. Sc. Vol. 6, p. 2G3, pi. 11, fig. 11. 

 A. id. Kussel, Essex Jour. Nat. Hist, Vol. 1, p. 52. 



Cochlodesma leana. Couthouy, Bost. Jour. Nat. Hist. Vol. 2, p. 170. 

 C. id. Gould, Invertebataof Mass. p. 49, figs. 27, 30. 



Description. Shell very thin and fragile, ovate, subcompressed ; the left valve almost flat, 

 rounded at both ends ; the right valve convex, and subtruncate at the shorter end, slightly 

 gaping at both ends. Beaks small, slightly cleft at one side : from the beaks proceeds a ridge, 

 more or less obvious to the posterior end. Surface wrinkled, with a yellowish shining epi- 

 dermis extending sowewhat beyond the margins ; the spoon-shaped process in the hinge nearly 

 horizontal, and resting on an oblique rib directed backwards : no ossiculum. 



Color, white beneath the epidermis. 



Vertical axis, 0*9; transverse ditto, 1*3. 



This is found occasionally along our coast, and is said to be very abundant about Cape 

 Cod. The flattened valve is frequently eroded in the centre. 



GENUS THRACIA. Leach. 



Animal resembling Anatina. Shell usually thin, transversely oval, inequivalve ; right valve 

 most convex, slightly gaping at both ends. Beaks well marked, and inclined a little back- 

 wards. Tooth represented on each valve by a more or less prominent spoon-shaped process. 

 Occasionally a cylindrical and semicircular ossiculum is attached to the posterior extremity 

 of the internal ligament. Palleal impression deeply excavated behind. 



Obs. This genus was first established by Leach, and has been subsequently more amply 

 developed by Deshayes, and also by Mr. Couthouy in his elaborate monograph of the Family 

 Osteodesmacea already cited above. 



