499 PALiEONTOLOGY OF NEW-YORK. 



Leptoctrlia dicliotoiiia ( n. s.). 



Plat« cm B. Fio. 3 a, b, e. 



Shell oval ovate, concavo-convex. Ventral valve arcuate, strongly convex, 

 or approaching to subcarinate in the middle and abruptly sloping at the 

 sides : beak incurved. Dorsal valve concave, more abruptly depressed 

 in the centre, and flattened on the cardino-lateral margins. 



Surface marked by dichotomizing plications ; the central one of the 

 dorsal valve becoming tripartite, and the three lateral ones, which are 

 simple at their origin, bifurcating and making six at the margin of the 

 shell : on the ventral valve there are two smaller plications in the 

 centre, and four dichotomizing ones on each side ; concentrically 

 marked by a few imbricating lines of growth. 



This species resembles L. concava of the Lower Helderberg rocks, but is much 

 larger, and the manner of bifurcation of the plications is very distinctive. The 

 depressed space in the centre of the dorsal valve is not so abrupt, and the flattened 

 portion on the dorso-lateral margins is proportionally much narrower than in the 

 species from the limestone. 



Fig. 3 a. Dorsal view of the specimen. 



Fig. 3 b. Ventral view of the same. 



Fig. 3 c. Front view, showing the outline, the depression of the dorsal valve, and 

 the incurved beak. 



Geological position and locality. In the Oriskany sandstone : Cumberland, Md. 



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Fig. 1. The ventrkl valve of L.ftabtllittt, showing the dental processes and the muscular impression. 

 Fig. 2. Tlic dorsal valve, showing the cardinal processes, dental fossets, muscular impressions, etc. 

 Flg.3. Dorsal view of i.^«i6r<o<o. 



