No. 109.] 



93 



It is closely related to Terehratula comprcssa ( Murchison), but at- 

 tains a larger size than any of ttat species figured, and is almost always 

 more gibbous, especially the ventral valve near the beak. 



A single specimen, apparently of this species ( pi. 40, f. 2 c, d, e & f ), 

 is much more compressed than the others, and less distinctly sinuate on the 

 middle of the valves, and consequently nearly destitute of the emargination 

 in the front : this, however, is only a single exception to the general cha- 

 racters of the species. 



Geological position and locality. Shaly limestone of the Lower Helder- 

 berg group, Albany county. 



MeRISTA BELLA. 



MeRISTA SUBQUADRATA (U.S.). 

 Pal. N.Y. Vol.iii, pi. 40, f. 3. 

 Shell subquadrate : ventral valve the larger, gibbous in the 

 middle and umbonial region ; beak prominent, incurved, ap- 

 parently not perforate : dorsal valve depressed-convex ; front 

 slightly elevated, forming a small undefined mesial prominence 

 immediately on the margin; beak well defined, incurved. Sur- 

 face smooth, or marked with many indistinct concentric lines of 

 growth. 



The most marked characters of this species are its obliquely subquadrate 

 form, and the slight elevation of the front margin of the dorsal valve, with- 

 out any traces of a corresponding sinus in the opposite valve. 



Geological position and locality. Shaly limestone of the Lower Helder- 

 berg group, Albany county. 



