Weller — Kinderhook Faunal Studies. 183 



inflated, without a sinus, covered with about thirty rugose 

 ribs. Ears small, scarcely separated from the body of tbe 

 shell. Beak rather large ;<nd recurved. Ribs flattened and 

 crossed by fine lines, many of them dichotomous. Area and 

 brachial valve unknown. Traces of tubes can be seen on the 

 cardinal edge, but the number cannot be ascertained. Length 

 6 mm. ; breadth 9 mm. 



Remai'ks. This species h is always been the cause of much 

 confusion. Only a few years after its first description by 

 Norwood and Pratten, Hall* identified a common species from 

 the Burlington limestone :ts C. logani, which he described as 

 having from 100 to 125 dichotomizing striae, while the original 

 C. logani was said to have but about 30. Worthen t first 

 detected Hall's error and gave the name C. illinoisensis to 

 the Burlington limestone species. At the time of publication 

 of volume IV of the New York Paleontology, Hall J seems 

 to have recognized the true C. logani from Burlington. The 

 latest reference to the species has been made by Girty,§ who 

 has described it from the Madison limestone of the Yellow- 

 stone Park, referring it to Shumard's species C omatus, 

 although the identity of the species with C. logani is sug- 

 gested. The specimen illustrated by him is as typical C. logani 

 as any that can be found at Burlington. 



At Burlington this species seems to be restricted to the 

 oolitic limestone bed No. 6. It can always be recognized by 

 its rather coarse plications and by its concentric markings 

 which are stronger on the ribs than in the depressions. The 

 shell is also more convex than any of its associates, and the 

 fullness extends well out towards the cardinal extremities so 

 that the auriculations of the shell are not so conspicuous as 

 they are in some members of the genus. The shell often 

 attains a greater size than the dimensions given by Norwood 

 and Pratten, the larger ones being 8| mm. long and 10 mm. 

 wide. The laro;er shells are more convex than the smaller 



* Rep. Geol. Surv. la. I 2 : 598. 



t Trans. Acad. Sci. St. Louis. 1 : 571. 



J Pal. N. Y. 4: 137. 



§ Monog. U. S. G. S. 32: 527. 



