TRENTON LIMESTONE. 147 



elevation scarcely distinct, and the sinus short, extending less than half way to the beak ; the 

 plaits arc 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 fiexuous imbricating strice, 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 fiexuous lines. 



^'S- '3 oi ^! *• 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 L A small specimen of the western type, corresponding very nearly with 13 a in size and form. 



Fig. 13 /j 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 ;>, 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 ernarginate in front, from the extension of this portion of the dorsal valve. 



Fig. 13 t, n. 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, fillmg the deep emargination of the opposite valve. 



Fig. 13 y. Cardinal view, showing ihe 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 New-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 througli several hundred feet in thickness. It occurs at JMiddlevillc, 

 Trenton Falls, Herkimer, Turin, Watcrtown and other places in New-York. It is abundant 

 at Cincinnati and Oxford (Ohio), iMadison (Indiana), iMaysville and Frankfort (Ky.), 

 and numerous other places. (State Collection.) 



19* 



