1«4 PALAEONTOLOGY OF NEW- YORK. 



Pig. 17, 18. The interior of the dorsal valves of the broad variety, the last having the di- 

 mensions of an inch and a half by an inch and a . quarter. 



In this specimen the cardinal process is much thickened and elevated, so as 

 to fill the entire foramen of the opposite valve. 



Fig. 19, 20. Casts of the ventral valve, showing the muscular and vascular impressions. 



Pig. 21, 22. Casts of the dorsal valve, with impressions of the cardinal and brachial pro- 

 oesses. 



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

 group : Helderbei^ mountains; Schoharie, Carlisle, Cherryvalley, Catskill, Hudson, 

 etc. 



Oithis oblata, var. euiarginata. 



Plate X A. Fio. 4-6. 



Among the collections from the Lower Helderberg rocks of Cumber- 

 land, Maryland, there are numerous specimens of an Orthis, having the 

 same general characters as O. oblata, but proportionally narrower and the 

 beak more extended. The dorsal valve is very convex, and the ventral 

 valve is marked by an undefined depression, which, beginning below the 

 beak, gradually becomes wider and deeper, producing a deep sinuosity or 

 emargination in front. 



The vascular area of the ventral valve occupies a comparatively greater space 

 than in shells of 0. oblata from the Helderberg, and it is more elongated or tri- 

 angular in form. In the dorsal valve, the double imprints of the adductor muscles 

 are well preserved. 



Fig. 4 a, h, c, d. Illustrations of the form and characters of this variety. 

 Fig. 5 a, h. Interior of the two valves. • 



Fig. 6 a, h. Cast of the ventral and dorsal valve. 



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

 group : Cumberland, Maryland. 



