TRENTON LIMESTONE. 103 
the compact limestones of New-York, where, 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. The young ones, or those 
of medium size, are usually symmetrical shells, moderately convex, and gently curving 
towards the base. The striz 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 striz appear of uniform size, 
or alternate irregularly. The concave or ventral valve, particularly in old specimens, has 
the striz nearly uniform in size. 
Pratz XXXI. 
Fig. 1 a, A specimen of medium size, with the elevated striz very distinctly marked, and the gradual 
enlarging of one of the intermediate smaller strie is shown as they recede from the apex. 
Fig. 1.5. A smaller specimen, presenting the characters of the species in a very perfect manner. 
Fig. 1 c. 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 striae 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 striz alternate 
precisely in the same manner with four to six smaller ones, the whole being crossed by fine 
concentric slightly undulating lines. 
Fig. 1 f. A specimen having almost precisely the same form as 1 e; but the larger striz 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 striw 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 strie very strongly marked : a few imbricating lines of growth are visible 
near where the curvature of the shell becomes abrupt. 
Fig. 1 4. Ventral valve of another specimen; the radiating strie 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. 
