216 



tions included in the sinus. Dorsal valve a little the 

 more gibbous, somewhat flattened forward of the umbo ; 

 three or four of the plications continuing direct, and 

 forming a mesial elevation, the lateral ones arching down- 

 wards to the margin. Shell marked by from nine to 

 twelve strong, rounded or subangular plications, which 

 sometimes become obsolete towards the beak. The con- 

 centric lines are very obscure. Length about half an 

 inch. 



This species resembles the R. neglecta of the Niagara 

 group of New York; but is larger and more robust, with 

 plications stronger and more rounded. It is associated with 

 a more finely plicated species which I have identified with 

 the R. neglecta from the Niagara shale of New York. 



Rhynchonella whitii {Hail). 



Shell subcircular or transverse, and broadly elliptical, 

 usually not very gibbous. Ventral valve shallow, most 

 prominent on the umbo ; beak abruptly attenuate and 

 pointed, slightly incurved, sides flattened, strongly sinu- 

 ate in the middle, and gently curving upwards in front. 

 Dorsal valve more gibbous, beak obtuse and incurved 

 beneath the opposite, elevated in the middle, forming a 

 distinct mesial fold, arching on the sides. There are from 

 five to six strong plications on each side of the mesial 

 fold and sinus, while there is usually a single plication in 

 the sinus, and two on the mesial fold. Rarely one of the 

 plications on the fold is bifurcate. The interspaces are 

 wider than the plications. Surface marked by close con- 

 centric lines of growth. 



This species bears some resemblance to R. (Jitrypa) dentata 

 {Pal. N. F., vol. I, p. 148), but is less gibbous, and the pli- 

 cations less strong, the mesial fold is less elevate'd, and the 

 sinus much narrower. 



