TRENTON LIMESTONE. 103 



the compaci limestones of New- York, wliere, in splitting the rock, a portion of the shell 

 often exfoliates, we find a great deviation from the type of the species. The age of the 

 shell also exerts considerable influence upon its external form. Tiie yotmg ones, or those 

 of medium size, are usually symmetrical shells, moderately convex, and gentlj- curving 

 towards the base. The stria; in such individuals are distinctly in fascicles of four to six, 

 separated by stronger and more elevated single ones on either side ; as the shell increases, 

 this uniformity in the arrangement often disappears, and the strioe appear of uniform size, 

 or alternate irregularly. The concave or ventral valve, particularly in old specimens, has 

 the striae nearly uniform in size. 



Plate XXXI. 



Fig. 1 a, A specimen of medium size, with the elevated strioe very distinctly marked, and the gradual 

 enlarging of one of the intermediate smaller strice is shown as they recede from the apex. 



Fig. 1 i. A smaller specimen, presenting the characters of the species in a very perfect manner. 



Fig. 1 e. The inside of the dorsal valve of this species, showing, imperfectly, the visceral impression 

 near the beak, and the papillose surface of the shell. The alternation of larger stri.-B with 

 fascicles of smaller ones is quite distinct. 



Fig. 1 d. An individual of larger size, having the essential features of the species well preserved. This 

 specimen is much more convex in the centre, and more abruptly curved in front, than either 

 of the preceding. 



In the four figures above, the width considerably exceeds the height of the shell ; but this 

 character is not constant. 



Fig. 1 e. The dorsal valve of another species; the length and breadth are about equal, and in this respect 

 only does it differ from the preceding ones. The larger and more elevated striae alternate 

 precisely in the same manner with four to six smaller ones, the whole being crossed by fine 

 concentric slightly undulating lines. 



Fig. I /. A specimen having almost precisely the same form as 1 e ,• but the larger striae are not .so 

 prominent, and the intermediate smaller ones not as regular as in the last species. This 

 character, taken in connexion with the deviation in form, has been regarded as indicating a 

 distinct species ; but it is clearly identical with those previously noticed, showing no greater 

 difference in regard to the strire than is observed in others of the same form as 1 a, b, which 

 will be shown on the succeeding plate. 



The foregoing figures represent the principal forms found in the Trenton limestone in 

 the State of New- York. In western localities of rocks of the same age, this species assumes 

 a greater variety of form, and has received several distinct names. 



Fig. 1 g. The dorsal valve of this species, from the shaly blue limestone of Ohio. The surface is well 

 preserved, and the stria; very strongly marked : a few imbricating lines of growth are visible 

 near where the curvature of the shell becomes abrupt. 



Fig. 1 h. Ventral valve of another specimen ; the radiating striae are nearly equal, the larger elevated 

 ones, dividing the fascicles of small rays, being scarcely distinct, while on the dorsal valve 

 they present the same distinction as in the last figure. The hinge view, of the same shell 

 shows the deltoid foramen nearly closed, with the small perforation in the beak, which is 

 enlarged in the upper figure. 



