LAMELLIBRANCHIATA. 539 
Cyrtodonta 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 this is still to be regarded as a well 
marked species. Itmay be compared with C. huronensis Billings but will be found 
to be higher, more erect and more ventricose. The umbdnes also are larger and 
the cardinal teeth longer and more nearly horizontal. C. obliqua Meek and Worthen 
has a straighter basal line and is more produced in the postero-ventral region. C. 
glabellus 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 
ventricosus Hall, sp., in 1882 (loc, cit) is not identical with C. billingsi, 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. K. 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 Palearca. This species is not the 
same as C. billingsi being longer and having a well developed legamental area and 
