236 UNITED STATES GEOLOGICAL SURVEY OF THE TERRITORIES. 



L. (Cymella) undata (zr Pholadomya undata, M. & EL), and L. (Cymella) 



bella, Conrad, -of this country; also, L. (Psilomya) lata, and^Z,. (Psilomya) 



globosa (— Poromya lata, and P. g/obosa, Forbes), us well as L. (Psilowi/a) 



mperba (=z Poromya superba, Stoliczka), from the Cretaceous of India. As 



remarked by Mr. Conrad, Cardium subdinnense, and C. Cornulinianum, 



d'Orbigny, from the Cretaceous rocks of France, have al least the external 



characters of Liopistha proper, and probably belong to this group; though 



their hinge-characters may possibly be found different, when they can be 



made out. 



In regard to (lie family-affinities of Liopistha, 1 would merely remark, 



that, notwithstanding its apparent relations to Poromya, which nearly all 



authorities place in the Corbulidce, it seems to me to belong more properly in 



the Anatinidce, so far as can be determined from all of its characters yet 



known. 



Subgenus CYMELLA, Meek. 



Liopistha (Cymella) undata, M. & H 



Plate :», figs. 1, «, b. 



Pholadomya undata, Meek and Haydeu (1856), Proceed. Acad. Nat. Sci. Pbilad., VIII, SI. 



Pholadomya {Cymella) undata, Meek (1864), Smithsonian Check-List N. Am. Cret. Invert. Foss., 14 and ;!4. 



Shell transversely broad-ovate, approaching subtrigonal, moderately 

 gibbous; anterior end rounded; posterior side narrower and a little more 

 compressed, rounded chiefly from below; base forming a regular semi-ovate 

 curve; dorsal margin sloping rather abruptly in front of the beaks, straighter 

 and declining more gradually behind ; hinge-margins straight, and inflected 

 so as to form a well-defined false area both behind and a little in front of 

 the beaks, which are somewhat elevated, incurved at right angles to the 

 hinge-line, and located a little in advance of the middle of the shell. Surface 

 ornamented by about seventeen to twenty of the simple, rounded, rather 

 strong, regular, concentric undulations, which are broader than the depres- 

 sions between, and, as it were, cut by the radiating linear furrows, on the 

 central region of each valve, into about the same number of much smaller, 

 simple, radiating costae, less than, or nearly equaling, the furrows by which 

 they are separated. 



Length, 0.S5 inch; height, 0.76 inch; convexity, 0.48 inch. 



The radiating costae occupy a triangular area on the middle of each 

 valve, covering more than one-third of the entire surface, while the remaining 



