«88 PALiEONTOLOGY OF NEW-YORK. 



Rliynclioiiflla veiitricosa ( u. s.). 



Plate XLIII. Fio. 1 a - m. 



Shell varying from Bpheroidal to vertically ovoid, extremely ventricose ; 

 depth of the valves often nearly double their breadth : valves nearly 

 equal in the young state. Dorsal valves sometimes a little depressed in 

 the umbonial region ; beak incurved ; cardinal region on each side of 

 the beak elevated. Ventral valve having a long linguiform extension : 

 beak rather small and obtuse, closely incurved upon the opposite, sub- 

 angular along its lateral slopes. 



Surface marked by fourteen to twenty rounded plications, three or four 

 of which are sometimes very slightly elevated on the middle of the 

 dorsal valve, so as to form an extremely obscure mesial fold, and two 

 or three as much depressed on the ventral valve : plications on the 

 front marked with a longitudinal depressed line, and remains of much 

 finer closely arranged zigzag lines of growth. 

 In the young shell the two valves are nearly equal ; but as it grows older, the 



dorsal valve becomes more ventricose, attaining its maximum in fig. 1 i. 



This species belongs to the type of R. wilsoni, and is one of those forms which 



are often referred to that species. In some of its varieties it resembles R. pyramidafa ; 



but that species is more angular on the sides, and never so abruptly sloping to the 



beaks. In some of its phases it more nearly resembles the R. nucleolata in form, but 



the plications are stronger. 



Among many hundreds of individuals of the preceding species collected from 



the Shaly limestone, I have not seen one of this species, and it apears to be restricted 



to a bed in the upper part of the group. 



Fig. 1 a, b, c. Dorsal, Tentral, and profile views of a specimen of ordinary size and pro- 

 portions. 



Fig. 1 d. Front view of the same specimen. 



Fig. 1 e,f, g, h. Dorsal, ventral, and profile views of more ventricose specimens. 



Fig. 1 t, k. Profile and front views of an extremely ventricose specimen. 



Fig. 1 /. Dorsal view of a more elongate form than usual. 



Fig. 1 m. Enlargement of the plications, showing impressed lines and strong striae in the 

 front of the shell. 



Geological position and locality. In the upper pentamerus limestone of the Lower 

 Helderberg group : Schoharie, Carlisle and Cherryvalley. 



