444 PROCEEDINGS OF THE NATIONAL MUSEUM. vol.49. 



dorsal area is relatively narrower, bounded by a rib, beyond which 

 is another more anterior rib, while in the left valve there is an obscure 

 ray about the middle of the disk, but hardly visible in the opposite 

 valve; at the posterior end the truncation is but obscurely indicated; 

 the base is prominently arcuated. Length, 17; height, 13; diameter, 

 9 mm. ; vertical of the beaks 7 mm. in front of the posterior end and 

 barely reached by the paUial sinus. 



Distribution. — ^Type locahty, Gulf of California. Cat. No. 73639, 

 U.S.N.M. 



THRACIA SQUAMOSA Carpenter. 



T. squamosa Carpenter, Proc. Zool. Soc. London, for 1855, p. 229. Type local- 

 ity, Mazatlan, Mexico. — Reeve, Con. Icon. Thracia, pi. 3, fig. 16, 1859. 



This species has much the form of T. phaseolina, but the surface 

 is densely granulose; ligament inclosing the resilium on the hinge 

 margin; there is a vertical scar under the beaks, very large and 

 more conspicuous than in most bivalves if, as seems probable, it 

 denotes the attachment of the lithodesma. No lithodesma was 

 noticed in the specimens. 



Distribution. — ^Magdalena Bay, Lower Cahfornia, to the Gulf of 

 California. Cat. No. 15885, U.S.N.M. 



Genus CYATHODONTA Conrad. 



Cyathodonta Conrad, Proc. Acad. Nat. Sci. Phila., vol. 4, p. 156, 1849. Type. — 

 C. undulata Conrad. 



CYATHODONTA UNDULATA Conrad. 



C. undulata Conrad, Proc. Acad. Nat. Sci. Phila., for 1849, vol. 4, p. 156. 



C. granulosa Gould, Boston Joum. Nat. Hiat., vol. 6, p. 407, 1852, name only. 



Cited by Carpenter in Rep. British Assoc. Adv. Sci. for 1856, p. 231. 

 C. plicata Carpenter and others, not of Deshayes. 



This shell is large, elongate, with a very flat left valve and more 

 convex right valve; the plaits are hardly oblique, and as usual are 

 obsolescent on the part of the valve behind the anterior ray. They 

 number 18 to 20. There are three faint rays on the left valve and 

 two on the right. The surface is covered with minute granules 

 definitely arranged in lines radiating from the umbones. The pallial 

 sinus is rounded and high, reaching the vertical from the beaks, 

 which are subcentral. Length, 48; height, 35; diameter of right 

 valve, 10; left, 4 mm. The vertical from the beaks is 26 mm. in 

 front of the posterior end of the valves. 



Distribution. — ^La Paz and other localities in the Gulf of California. 

 Cat. No. 73602, U.S.N.M. 



This fine species is the largest of the West Coast species and the 

 analogue of the C. magnijica Jonas, from the Atlantic coast of Hon- 

 duras. Still larger single valves are in the collection. 



