aea palaeontology of new-york. 



Fig. 1 n, 0, p. Ventral, profile and front views of a large specimen which preserves the 

 prevailing character of the species. The profile view shows the incurved dorsal 

 beak, and the nearly straight ventral beak, a little separated. 



Fig. 1 r. Cardinal view, showing the short accessory plications which mark the lateral areas. 



Fig. 1 s. Profile view of another spooimen, showing the beaks of the two valves in contact, 

 the usual condition of the shell. 



Fig, 1 /. Interior of the dorsal valve, showing the two longitudinal septa and the broad 

 brachial processes at the hinge line. 



Fig. 1 K, Interior of the ventral valve, showing the elongate spoonshaped cavity, with the 

 lateral lobes and the slight extension of the central septum. 



Fig. 1 I. The same enlarged, showing the muscular imprints at the base of the cavity. 



Fig. 1 p. Interior of the two valves in connexion, showing the triangular cavity and central 

 septum below, and the double septa above with the lamellas, which extend far 

 into the interior of the upper or dorsal valve. 



Fig. 1 y. Longitudinal section of the ventral valve, showing the comparative extent of the 

 spoonshaped cavity and the narrow short septum below. 



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

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



Peiitaiuerus littoiii ( n. s.). 



Shell ovoid, spmewbat elongate. Dorsal valve moderately and regularly 

 convex from base to umbo ; beak almost rectangularly incurved beneath 

 the beak of the opposite valve. Ventral valve very gibbous ; beak sub- 

 attenuate, incurved. 

 ScKPACE marked by about eighteen or twenty simple subangular plica- 

 tions. There is a broad concave smooth space on each side below the 

 beak of the ventral valve, and a much narrower space on each side of 

 the dorsal valve. 



In a specimen which is entire with the exception of the beak of the ventral valve, 

 the greatest depth of the two valves together, which is about the middle of the 

 shell, is equal to the greatest width, which is at a point much nearer the base. 



I am indebted to Dr. Litton, of St. Louis, for this species, which was collected 

 by him some years since in Hardin county, Tennesse*, associated with Pentamerus 

 galeatus and other fossils of the age of the Lower Helderberg group. 



