TRENTON LIMESTONE. 135 



plication, which is intermediate to the mesial or lateral series ; and the lateral plaits, in 

 this case, are six on one side and seven on the other. 



In one individual (fig. 1 r), approaching to S. chama, and having but one fully developed 

 plication in the sinus and two on the mesial lobe, the lateral ones are nine and ten. In two 

 other specimens resembling the last, where the mesial plates are better developed, the outer 

 ones being still smaller, the lateral plications are eight on each side. In two other some- 

 what similar specimens, the lateral plications are six on each side ; while in a single smaller 

 one, there are but five plaits on each side of the middle lobe and sinus. 



In the six specimens here enumerated, the shell is more distinctly trilobate ; the sinus 

 deeper and more elevated in front, lifting up the mesial lobe higher in this part than it is 

 in the centre of the shell, thus contrasting with the prevailing forms before noticed. 



These forms constitute a variety, which, though easily traced to the prevailing form, is 

 nevertheless somewhat constant, and worthy of attention, being the nearest approximation 

 to the Var. B. Sp. dentatus, which occurs in Russia. 



It has only been possible to illustrate the characters of this species by a great number of 

 figures, drawn from a selection of specimens, both of New-York and the Western States, 

 which exhibit in the best manner the gradations in form, and other characters which this 

 truly protean species presents. 



Figs. 1 a, b, c, d. A series of specimens, showing the gradations in size, and the increase in number of 

 plications as the shell becomes older. The smaller specimen 1 a has three plaits in the sinus 

 and four on the mesial lobe, with seven on each side, thus : 7 f 7. [n fig. 1 b, there is the 

 partial development of a fourth plication in the sihus, and a fifth on the mesial lobe, though 

 there is no increase in the number of lateral ones. In figs, c and d, there is a full develop- 

 ment of the I medial plications, and 10 lateral ones; the last specimen being about the size 

 of this type in the New- York rocks. 



Figs. 1 e and / are front views of a and d. 



Fig. 1 g. Profile view of the specimen d. 



Fig. 1 k. Ventral valve of another form, showing extended beaks. Medial plications a ; lateral ones 13 

 on each side. 



Fig. 1 t. Front view of the same. 



Figs. 1 k, I, m, 0, q, s, u. A series of western specimens, with the mesial and lateral plications ( 7 f 7) ; 

 showing no tendency to increase of either number, though the size of the shell increases as 

 in the previous series. Figs. 1 m, q and *, show the cardinal extremities more extended and 

 acute than in the other forms; while the specimens o and « have the cardinal extremities 

 shorter, scarcely equalling the width of the shell. 



Figs. 1 71, p, r, t, are front views of figs. 1 m, o, q, s and u, respectively. 



Fig. 1 A, B. Cardinal and profile view of an extremely globose specimen, having the cardinal line much 

 less than the width of the shell, and the extremities rounded. The area is broad, and very 

 well definad. The plications are 7 *• 7. 



Fig. I c. Dorsal valve of a specimen, having the cardinal line extended beyond the width of the shell, 

 and terminating in short acute ears. 





