98 PALAEONTOLOGY OF THE UPPER MISSOURI. 



concentric markings sometimes very fine or obsolescent). Muscular impressions 

 distinct ; pallial line somewhat sinuous. 



This group is nearly related to some of the sections of Cardium, but may be 

 generally readily distinguished by its peculiar surface-sculpturing and slightly sinuous 

 pallial line. Still, as its principal difference from forms referred by many to the 

 genus Cardium consists in its surface-markings, most palaeontologists regard it as 

 forming only a subgenus under that group. Although we have no very serious 

 objections to this conclusion, we think it more properly constitutes a distinct genus 

 from Cardium as typified by C. costatum, Lin. Although it can nearly always be 

 identified by its sculpturing alone, there are a few Cretaceous species in which these 

 markings are very faintly defined, or probably in some case entirely obliterated. 

 The radiating costae or striae on the posterior side of the valves, however, are nearly 

 always present, even when the concentric sculpturing on the sides and front are 

 obsolete. Very rarely, however, the radiating markings are obsolete ; even in these 

 cases, however, some traces of their existence can be seen in the crenulated margins 

 of the posterior side of the valves. 



This genus seems to be entirely confined to the Jurassic and Cretaceous rocks, 

 unless a few recent shells, such as Cardium pectenalum, Lin. (not Lamk.), and C. 

 lyratum, Sowerby, from the Philippines, may belong to the same group. These two 

 species agree in form, and have very nearly the surface-markings of Protocardia, and 

 unless they present some differences in the hinge or interior (we are not acquainted 

 with the interior of these shells), they must be nearly related to the group under 

 consideration. Still, they differ in having the sculpturing on the sides and front of 

 the valves, somewhat oblique, and intersecting the anterior margins, instead of 

 concentric and curving upwards in front. They were referred, by Swainson, Morch, 

 Chenu, Adams, and others, to Liocardium, but they differ materially from C. Icevi- 

 gatum, usually regarded as the type of the la^er group. 



Protocardia Shumardi. 



Cardium Shumardi, MEEK & HAYDEN, Proceed. Acad. Nat. Sci. Phila. May, I860, 182. 

 Cardium (Protocardium?) Shumardi, MEEK & HAYDEN, ib. 418. 

 A B 



CD 



Protocardia Shumardi. 



A. Side view of internal oaat. B. Outline front view of same. 



Shell small, oval snbcordate, rather gibbous ; anterior side rounded ; base more broadly rounded ; posterior side 

 obliquely subtruncate above, and intersecting with an abrupt curve, the base below. Hinge margin rather short, 

 and sloping slightly from the beaks, which are moderately elevated, gibbous, and nearly central ; posterior umbonal 

 slopes snbangular. Surface of oasts retaining only traces of small radiating costas, or lines, on the prominent 

 posterior umbonal slopes and the flattened postero-dorsal region. (Hinge and interior unknown.) 

 Length, 0.44 inch ; height, 0.37 inch ; thickness, 0.32 inch. 



Our specimens of this species being casts, it is impossible to determine whether 

 or not the surface was marked by regular concentric stria? on the middle and 

 anterior portions, though it probably was. In its general appearance it bears some 



