INVERTEBRATE PALEONTOLOGY. 89 



This species will be al once distinguished from the following by its much 

 larger cardinal area, more depressed and oblique form and warped base, as well 

 as by ils more prominently-rounded umbonal slopes. It differs from old 

 specimens of C Shumardi in being more depressed, in having a larger area 

 and a thicker shell, as well as in its warped pallial border. 



In some respects, this species would conform more nearly to the group 

 Latiarca, but its distinctly-projecting internal plate, connected with the pos- 

 terior muscular scar, would forbid its association with that section of the genus. 

 The two groups, however, are not always strongly distinct, I think. 



Locality and position. — Fox Hills and Moreau River; from same position 

 as the last. 



< • u <■ ii ! I :v :i (Idonearca!) cordata, M. & H. 



Plate 29, figs. 6, a, b. 



Area (Cuoullaia) cordata, Meek and Hayden (Apr., 1858), Proceed. Acad. Nat. Sci. Philad., VIII, 86. 

 Ciicnllaa cordata, Meek and Haydeu (Nov., 1856), ib. t 285. 



Shell rhombic-cordate, very gibbous, apparently equi valve; pallial border 

 smooth within; anterior side short and rounded; base forming a semi-ovate 

 curve, the most prominent part of which is near the posterior extremity ; pos- 

 terior side obliquely truncated with a slightly convex outline above, narrowly 

 rounded below; beaks elevated, very ventricose, incurved nearly at right- 

 angles to the hinge, located in advance of the middle ; cardinal area short, 

 being but little more than half the length of the shell, marked by numerous 

 very fine, straight, longitudinal striae, and having on each valve seven or eight 

 strong, oblique grooves, which are deflected so as to form angles of 150° under 

 the beaks. Surface marked by distinct lines of growth, crossed by very 

 obscure radiating costae. 



Length, 2.10 inches; height from base to top of umbones, 1.95 inches; 

 convexity of the two valves, 1.78 inches; length of ligament-area, 1.25 inches, 

 breadth of same, 0.30 inch. 



It is probable that in young individuals the radiating costse are more 

 strongly marked; while on old shells, or those that have been slightly worn, 

 they appear to be generally obsolete. The most convex portion of the valves 

 is along the posterior side of the umbonal slopes, which arc quite prominent, 

 but not so much so as to form ridges. < *u the anterior side, there is an irrejj- 



12 ii 



