PLATE 31. 



Fig. 1 a -/, & 2 a - y. Rhynchonella nucleolata. 227 



1 a, b, c. A young individual and one of medium size, having smaller plications than usual, 

 and a larger number upon the mesial sinus. 



1 d, e.f. Larger individuals having stronger plications than the preceding, and a larger num- 



ber upon the mesial fold and sinus than those which follow 



2 a - e. Figures presenting the ordinary character of the smaller individuals of this species. 



2/- t. Illustrations of the gradations in size, form, and proportions of this species. 



2 u, X, y. A cast of this species, showing the form of the muscular impression in the ventral 

 valve, the median plate of the dorsal valve, and the alveolar cavities. 



Fig. i a - d. Rhynchonella abkupta. 228 



3 a, b. Dorsal and ventral views of a well-marked individual of this species. 

 3 c, d. Front and cardinal views of another individual. 



