>OKTA KKriRVA I'lricli. 

 IK \i it } iti* - 

 /v//it>m|ru rrmrtvi UI.HK-II, IHM. Nineteenth An ai. 



Shell >mall or of medium >i/.e, compressed-convex, slightly oblique. Militriangu- 

 lar. the length and bight almost equal, the thickness equalling alout one-third of 

 the bight. Knstral portion strongly recurved, ends narrowly rounded, base nearly 

 -emi-elliptical. with more curvature in the posterior half than in the anterior. 

 IJeaks prominent, posterior to the center of the shell; umbones with an mutually 

 -ma II degree of convexity. Cardinal margins sharply inflected, forming an elongate 

 depressed area on the anterior side and a shorter, narrowly cordiform one behind or 

 rather li.-m-ath the beaks. A shallow and gradually widening sulcus extends from 

 the beak along the antero-cardinal margin to the antero- ventral border. Surface 

 !>y several strong lines of growth and between them fine concentric stria-, 

 ten in 3 mm. Hinge plate strong, bent at a right angle, the posterior part 

 nearly straight (gently concave), somewhat shorter than the anterior, with about 

 twenty small, curved transverse teeth, decreasing, as usual, gradually in size and 

 curvature toward the beak; anterior part convex, with about thirty teeth. Consid- 

 ering the strength of the hinge plate, the teeth are very small. Immediately in 

 front of the beak, in the angle of the hinge, a narrow oblique space breaks the 

 continuity of the series of denticles. Hinge plate margined on the outer side by a 

 delicate sharp ridge; just within it a narrow furrow which has considerable width 

 and depth for some distance in front of the beak. In front of the beak and above 

 the marginal line of the hinge plate a small area is defined apparently for the recep- 

 tion of an external ligament (See note, p. 578.) Anterior and posterior muscular 

 scars distinct, though not very strongly impressed; as usual for this section of the 

 genus in size and form. 



The compressed form reminds of C. comprtusn, but the shape is different, the 

 length being greater, the ends more narrowly rounded and the rostral part more 

 strongly recurved. The surface markings also are coarser and the posterior lunettes 

 much letter defined, while a number of important differences may be observed in 

 their hinges. Hall's <'. altn is similar in the l>a>al part. Imt is a more convex shell 

 and much less curved in the rostral part. 



i.ifi'.n and locality. -Upper beds "? th- Hudson Hirer group, at MYral localltta* In Plllnon 

 n.unty. Mitiii. .M It In aMoclated with th- n. xt tpecic* (C. ft"- 



-pecle* occur*, though *o far a* otwerrnl only In the condition f <-*U, alv 

 and at Richmond. Indiana. 



