No. 109.] 151 



The original specimens of this species arc in a thinly laminated black 

 shale, and much compressed. Other specimens in more calcareous shales 

 are more convex, and sometimes gibbous. There are also gradations in size 

 and number of striae, which incline me to regard this species as possibly 

 identical with C. scitula, notwithstanding the wide difiFerenccs of character 

 in many individuals. A large number of individuals are required for the 

 determination of this question. 



Geological position and loialily. In the black shales (Marcellus shales) 

 at the base of the namilton group, and in the higher shale of the same, 

 and in the Genesee slate which succeeds that group. 



Tropidoleptus CARINATUS. 



Strophomena carinata, Conrad : Ann. Geol. Rep. 1839, p. 64. 

 Leptcena laticosta, Hall, 1843. 

 Leptcena laticosta of Owen and others. 



Shell transversely oval; hinge straight, not crenulated, generally 

 a little less than the greatest breadth of the shell, rounded or 

 very obtusely angular at the extremities ; sides broadly rounded ; 

 basal margin slightly sinuous : dorsal valve concave, having a 

 shallow mesial sinus, which is broad in front but continues 

 above the middle of the shell as a narrow groove, not larger than 

 those between the other costse; beak very small, projecting be- 

 yond the hinge-line, straight or curving slightly outward; dental 

 process extended beyond the hinge-line : ventral valve convex, 

 slightly flattened and contracted toward the extremities, gibbous 

 above the centre and in the umbonal region; beak obtuse and 

 truncated by the foramen ; area distinct, variable in Avidth, 

 extending to the cardinal extremities, longitudinally striated, 

 limited entirely to the ventral valve ; foramen very broad ,i caching 

 to the beak and having a semicircular outline above, more or 

 less closed by the prominent dental processes of the opposite 

 valve. Plications usually simple and rounded, about eighteen to 

 twenty on each valve, the middle one on the ventral valve 

 being larger and more elevated than the others, so as to form a 

 small mesial fold or carina. Surface ornamented by very fine 

 distinct concentric striae, presenting under a magnifier a very 

 fine textile style of ornament : substance of the shell punctate 

 throughout. 



