TRENTON LIMESTONE. 
147 
elevation scarcely distinct, and the sinus short, extending less than half way to the beak ; the 
plaits are more angular, from a partial exfoliation of the shell. 
Fig. 13 c, d Ventral valve and profile view. The specimen more rotund, and the surface marked by 
fifteen plications; the medial lobe is more elevated, and the sinus extends nearly to the beak. 
The flexuous imbricating striae, in both these specimens, are nearly destroyed from ex¬ 
foliation of the shell. 
Fig. 13 e,f Ventral valve and profile of a more gibbous form, the valve being nearly circular or broadly 
oval, and the beak less exsert than in the last. The number of plaits is 16, and they are 
distinctly marked by the concentric flexuous lines. 
Fig. 13 g, h, i. Front, profile and ventral valve of a larger and more rotund specimen, having a deeper 
and more strongly defined sinus and medial lobe, and the beak of the dorsal valve closely 
curved over the ventral valve. The surface is marked by 16 plications, which, as in the 
preceding specimens, are defined entirely to the beak. 
This specimen is the maximum size to which those of New-York attain; but in the 
western localities, we are able to trace the species from its smaller forms through all the 
grades here presented, beyond which it assumes the more globose appearance shown in 
some of the following figures. 
Fig. 13 k. A small specimen of the western type, corresponding very nearly with 13 a in size and form. 
Fig. 13 l, m, n , o. Ventral valves and front views of two larger individuals, showing a gradation in size. 
Every possible intermediate form between 13 k and the following specimen may be found 
in almost every locality. 
Fig. 13 p, r, s. Ventral valve, profile and front view of a larger specimen, varying but little in its pro¬ 
portions from the preceding ones. The sinus is deeper and more strongly defined, and the 
ventral valve more emarginate in front, from the extension of this portion of the dorsal valve. 
Fig. 13 t, u. Profile and front view of a more gibbous specimen. 
Fig. 13 w. Profile view of a very gibbous specimen, which has become nearly globose, the thickness 
equalling the length of the shell. 
Fig. 13 x. Front view of the same, showing the sinus of the dorsal valve much extended and elevated in 
front, filling the deep emargination of the opposite valve. 
Fig. 13 y. Cardinal view, showing the sinus and medial elevation, extending nearly to the beaks of the 
shell. 
Specimens are frequently seen, where the gibbosity of the shell is much more extreme 
than in this one, the thickness being one third greater than the height of the shell. 
A series of this species, and of Delthyris lynx , showing all the variations in form and 
size, constitute a very interesting collection for the study of those changes in species of the 
Brachiopoda, induced by age and local influences. 
Position and locality. This species is found in Nevv-York, in the central and higher part 
of the Trenton limestone, very rarely in the lower part. It is found in all western localities 
of this rock, ranging through several hundred feet in thickness. It occurs at Middleville, 
Trenton Falls, Herkimer, Turin, Watertown and other places in New-York. It is abundant 
at Cincinnati and Oxford (Ohio), Madison (Indiana), Maysville and Frankfort (Ky.), 
and numerous other places. {State Collection.) 
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