146 PALEONTOLOGY OF NEW JEESET. 



the valves. Valves usually very ventricose, with strong, rather inflated and 

 enrolled beaks, situated near the anterior end and curved anteriorly, and 

 strongly angular on the back ; umbonal ridge distinctly and prominently 

 angular, and highly arcuate in its passage from the beaks to the postero- 

 basal angle of the valves. Cardinal border arcuate, the cardinal and basal 

 margins subparallel and nearly equally curved ; posterior margin squarely 

 truncate below and sloping toward the hinge-line above ; anterior end 

 sharply rounded, and deeply excavated beneath the beaks. Postero-cardi- 

 nal slope rather abrupt. Surface of the shell, as indicated on the casts, 

 smooth or marked only by concentric lines of growth. 



The extei'nal form of these casts is quite variable in different individ- 

 uals, dependent to some extent on the degree and direction of the compres- 

 sion which they have undergone. This also affects more or less the ventri- 

 cosity of the valves, which in their natural condition has been very great. 

 I have seen no specimens yet which afford any of the hinge characters 

 which will enable one to compare them satisfactorily with the recent species 

 of the genus Isocardia. But there is one feature which exists on all the 

 casts I have examined in which it differs from that genus, namely, a curved 

 gi'oove just in advance of the beaks in each valve, indicating a raised ridge 

 on the inside of the shell, extending from beneath the beak to the under 

 side of the hinge-plate analogous to that bordering the muscular imprint of 

 CucuUcea. Should this prove to be the case in well-preserved individuals 

 it woukl probably require generic separation, but the material which I have 

 seen is too imperfect to warrant that step at the present stage of the inves- 

 tigation. The casts of this species have usually been referred to VenieUa 

 Conradi by collectors, but when compared with those of that species they 

 are remarkably different. Dr. Morton's type specimen does not appear to 

 be in existence; but from his figure I should judge it had been flattened by 

 compression, thereby extending the beak and giving more than the usual 

 amount of slope to the postero-cardinal border. The surface stria which he 

 mentions, and which is faintly indicated on the figure, is most probably 

 imaginary, at least no other specimen showing it appears to have been seen. 

 Mr. Gabb, in his Synopsis, p. 104, in a foot-note under Cardita decisa, says 

 Dr. Morton's type of the species is not to be found in the collection of the 



