948 PAL^iONTOLOGY OF NEW-YORK. 



sufficiently extensive collections to determine fully these characters, or whether it 

 may be only another phase of that species in its young state. 



Geological position and locality. In theshalyand compact limestones of the Lower 

 Helderberg group : Albany, Schoharie and Herkimer counties. 



Mcrista bella ( n. s.). 



Plate XL. Fia. \ a- p. 



Shell varying from suborbicular or subquadrilateral to transversely oval, 

 usually somewhat broader than long, gibbous. Ventral valve a little the 

 larger, most convex near the umbo : beak prominent and closely in- 

 curved. Dorsal valve convex, gibbous in the middle and towards the 

 umbo : both valves marked with a small sublinear mesial sinus, that 

 of the ventral valve stronger than the other, the two often giving a 

 distinct emarginate outline to the front. 

 Surface smooth, or marked by faint concentric lines of growth, with 

 much fainter indications of radiating striae. 



This species is characterized by its symmetrical form and the distinctly emargi- 

 nate character of the front, caused by the meeting of the narrow mesial depressions 

 of the two valves. The sinus on the front of the ventral valve is always broader and 

 deeper than that on the other, giving a waved outline to the margins of the valves. 

 Some of the specimens appear to have a small open foramen in the point of the 

 beak, but which may be accidental. 



It is closely related to Terebratula compressa ( Murchison), but attains a larger 

 size than any of that species figured, and is almost always more gibbous, especially 

 the ventral valve near the beak. 



A single specimen, perhaps of this species ( Plate xl, fig. 2 c, rf, e &/ ), 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. I have 

 regarded this one as a distinct species; but as no other specimens have come under 

 my observation, I am inclined to consider it for the present as a variety of the M. 

 bella. 



Pig. 1 a - h. Dorsal, ventral, profile, front, and cardinal views of young shells. 



Fig. 1 I, k, I. Views of a larger specimen. The species rarely attains a larger size than 

 these figures. 



Fig. I m - p. Views of an unusually large specimen of this species. 



Geological position and locality. In the shaly limestone of the Lower Tlelderberg 



gcoup : Helderberg mountains; Schoharie, Carlisle, and other places. 



