LAMELLIBEANCHIATA. 497 



Ohonyohla undata. 



of the shell leaving a distinct depression in the casts. Muscular scars large, situated 

 about midway in the postero-cardinal half of the valve, the two lobes united by a 

 narrow neck, the upper one oval in shape and about one-third as large as the more 

 nearly circular lower one. 



The posterior extremity is more produced and more narrowly curved than in the 

 other species referred to this genus. 



Formation and locality.— Ijowei limestone of the Trenton formation at Minneapolis, Minnesota. 

 Mus. Meg. No. 5526. ' 



Clionychia undata Emmons. 



PLATE XXXV, FIGS. 21 and 22. 



Pterinea undata Emmons, 1842. Geol. Report. New York, p. 395. 

 Ambonychia undata Hall, 1847. Pal. New York, vol. i, p. 165. 



Shell subquadrate, cardinal margin long, straight, anterior side straight, nearly 

 vertical, curving sharply backward below into the gently convex base, which in its 

 turn curves rapidly upward into the broadly rounded posterior margin; antero- 

 cardinal angle about 85°, postero-cardinal angle about 115°. Beaks prominent, 

 attenuate, slightly incurved, with the umbones strongly convex, the anterior slope 

 very abrupt, the rapidity of the descent becoming gradually less in following the 

 margin around to the posterior extremity of the hinge, where it is very gentle; 

 cardinal slope concave, becoming s'trongly so and very abrupt in nearing the beaks. 

 Surface marked with broad concentric folds, which are strongest on the cardinal 

 and umbonal slopes and fade away gradually in curving around to^the anterior side. 

 Immediately beneath the beaks the anterior side of a good cast of the interior 

 presents a sharply defined lunule-like impression, which, having been occupied by 

 aa internal thickening of the margin of the valves, was scarcely indicated on the 

 exterior of the shell. Hinge plate narrow, muscular impressions undetermined. 



The above description is based upon the specimen illustrated on plate xxxv. 

 It presents no evidence of distortion and seems to be in every respect in a good 

 state of preservation. Comparing this example with Hall's description and figures 

 of the New York types of the species we observe that it differs in several particulars 

 that might be regarded as important. The outline is more nearly qiladrate, and the 

 convexity of the valves less, giving a form that deviates from the figures of the 

 New York specimen precisely as C. erecta does from G. lamellosa. Hall also mentions 

 the absence of a " definite lunette," while such an impression is distinctly present 

 in the casts of the Minnesota specimens. Despite these differences I am almost 

 confident of the specific identity of the latter and the types of the species, Ibecause I 

 am inclined to doubt the actual existence of the discrepancies noticed. 



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