PALAEONTOLOGY OF NEW-YORK. 
Mhynchoiiella ventricosa (n.s.). 
Plate XLIII. Fig. 1 a - m . 
Shell varying from spheroidal 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. pyramidata; 
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 Slialy 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, ventral, 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 i, Jc. Profile and front views of an extremely ventricose specimen. 
Fig. 1 l. Dorsal view of a more elongate form than usual. 
Fig. 1 m. Enlargement of the plications, showing impressed lines and strong striae in tho 
front of the shell. 
Geological position and locality. In the upper pentamerus limestone of the Lower 
Helderberg group : Schoharie, Carlisle and Cherry valley. 
