„ , , LAMELLIBEANCHIATA. 539 



Oyrtodouta biilingsi.] 



and passing gradually into the broadly and uniformly curved posterior margin; 

 basal line most prominent and strongly convex behind the center, in front of which 

 point it ascends rather rapidly with a much more gentle curve into the short, small 

 and sharply rounded anterior end. Umbones full, large and prominent, beaks 

 small and strongly incurved; umbonal ridge subangular near the beaks only, incon- 

 spicuous in a lateral view. Surface marked with concentric lines of growth. These, 

 with the exception of a few near the margin, are obscure in the material at hand. 

 Ligamental area very narrow. Hinge plate of moderate strength, with three slightly 

 curved and nearly horizontal cardinal teeth and two or three slender posterior 

 lateral teeth in each valve. Pallial line and anterior adductor muscle distinct, the 

 latter rather small and of obovate or subcircular shape; posterior adductor faintly 

 impressed, situated immediately beneath the lateral teeth. Internal umbonal 

 sulcus and ridge slightly developed but always distinguishable on good casts of the 

 interior. 



Although closely simulating several others bhis is still to be regarded as a well 

 marked species. It may be compared with C. huronensis Billings but will be found 

 to be higher, more erect and more ventricose. The umbones also are larger and 

 the cardinal teeth longer and more nearly horizontal. C. ohliqua Meek and Worthen 

 has a straighter basal line and is more produced in the postero-ventral region. C. 

 glahellus and C. persimilis have a more rounded outline and smaller umbones, C. 

 subovata is longer, wider in front, not so ventricose, and has smaller umbones. A shell 

 that is likely to prove more troublesome to separate than any of these is the Van- 

 uxemia decipiens. They are associated in the same strata at Minneapolis but when 

 good casts are available they may be distinguished at once by the higher position 

 and much greater sharpness of definition of the anterior muscular scar in the 

 Vanuxemia. 



It is possible that the Wisconsin species referred by Whitfield to Cypricardites 

 vew^mosMS Hall, sp., in 1882 (loc'cit) is not identical with C. biilingsi, because his 

 illustration, if correctly drawn, would indicate a distinct form. However that may 

 be it is quite certain that he had this species before him when he drew up his 

 description, since it is not uncommon at the localities mentioned by him. It is 

 certain also that neither the specimen figured by him nor the form now named 

 after Mr. E. Billings, the founder of the genus, are the same as the types of Hall's 

 Edmondia ventricosa (Pal. N. Y., vol. i, p. 155; 1847). Indeed they are widely dis- 

 tinct species the last having proved to be a true Whitella and not Cypricardites nor 

 Cyrtodonta at all. On page 271, Pal. N. Y., vol. iii, Hall figures another species of 

 Cyrtodonta which he refers to his ventricosa as a Palcearca. This species is not the 

 same as C. biilingsi being longer and having a well developed legamental area and 



