Maryland Geological Survey 361 



Description. — " Subtriangular, elevated in the middle of the lesser 

 valve, and profoundly depressed towards the base of the larger; surface 

 with about 50 rounded costse; about 17 of which in mature specimens are 

 on the elevated portion of the upper valve ; sides dilated and rounded on 

 their margins." Conrad, 18-il. 



" Shell transversely oval. Dorsal valve the larger, somewhat gibbous, 

 having a round undefined mesial fold : beak incurved ; cardinal border ex- 

 cavated in deep fossets on each side of the beak, for the reception of the 

 dental lamellae of the opposite valve. Ventral valve nearly flat, most ele- 

 vated near the beak, having a somewhat deep broad rounded sinus near 

 the front margin, which is prolonged into a rounded or subtriangular pro- 

 jection." Hall, 1859. 



Length 5 cm. ; width 6 cm. 



A study of New York material of these large, abundantly plicated, 

 strongly sinuate rhynchonelloids convinces the writer that there is but one 

 species represented. As a rule rhynchonelloids are variable shells and the 

 degree of development of the fold and sinus and plications represents 

 changeable features. The number of plications on these shells varies 

 between sixty and eighty-five, while those on the fold vary from twelve to 

 twenty-five. The merging of Hall's species was also noticed by Clarke, 

 since he says that " the distinction between C. ohlata and C. pleiopleura 

 Conrad is not well marked." 



Occurrence. — Oriskany Formation, Eidgely Member. Shriver 

 Ridge Hill, Cumberland, Collier's Eun. 



Collection. — Maryland Geological Survey. 



Genus UNCINULUS Bayle 

 Hall and Clarke ' characterize this genus as follows : " There are large 

 numbers of paleozoic rhynchonellas which are characterized by a full sub- 

 cuboidal or subpentahedral contour, a fold and sinus not sharply developed 

 except at the anterior margin, an abrupt anterior slope, sharply serrated 

 lateral margins of contact, and low surface plications, each of which, on 

 the front of both valves, is marked by a fine median line." Internally 



^Hall and Clarke, 1893, Nat. Hist. N. Y., Pal., vol. viii, pt. ii, p. 197. 



