393 



Meris. 



the dorsal valve, slightly less. . . . Ventral valve the more 

 convex; greatest convexity a little above the middle, abruptly 

 curving to the cardinal margin, and more gradually to the 

 front. A comparatively deep, broad sinus extends from the 

 beak to the base, forming one of the most conspicuous features 



of the species Umbo prominent General aspect of 



surface that of a smooth shell with a few strong lines or 

 varices of growth. There are indications of radiating strise, 

 and it is possible that specimens in a better condition of preser- 

 vation would show both radiating and concentric strias. The 

 form of this species is very similar to that of Meristella hella^ 

 of the Lower Helderberg group, but that species has a de- 

 pression on both the ventral and dorsal valves, while this 

 species has a fold on the dorsal valve. The subrhomboidal 

 form distinguishes it from any species of the Upper Helder- 

 berg groups. Formation and locality. Chemung group, near 

 Warren, Warren county, Pennsylvania. 



Meristella Isevis. See Merista Iwvis. ~F/. 



Meristella nasuta, Hall. {Atrypa nasuta^ Conrad, 1840.) 



HalUSik.ARtMr^. 



