814 PALAEONTOLOGY OF NEW-YORK. 



in that one are more oblique, and never so nearly in the same plane, but the aper- 

 ture is more nearly round. 



These differences are very perceptible in a comparison of the figures on Plates 

 Lv and Lvi. The peculiarities of form may serve to distinguish the casts of the two 

 si>ecies, while the undulating strise of the one under consideration will distiuguisli 

 t))em by the surface characters. 



Fig. 10. View of the spire of a large individual, showing the undulating striao and the 

 sinuoBities of the right side of the aperture. 



Fig. 11a. View of the aperture of a smaller specimen, which is a oast. 



Fig. 11 6. View of the aperture of a similar specimen. 



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

 group : Albany and Schoharie counties. 



Plat>eeras siiiuatuiii ( n. s.). 



Platb lv. Fig. 5 & 7 ; and Plate LVII. Fig. 2. 



Shell depressed, somewhat obliquely ovoid : volutions about three, con- 

 tiguous, the last one becoming very ventricose, a little flattened on the 

 upper side, and expanded laterally to the axis of the spire : aperture 

 broad campanulate ; margin deeply sinuate. 



Surface marked by fine concentric or transverse lamellose striae, and 

 stronger wrinkles or folds. The striae are abruptly bent backwards on 

 the upper dorsal side, and a deep sinus marks the labrum : the lower 

 side of the aperture is likewise deeply sinuate. 



The specimen fig. 5 is vertically compressed on the outer margin of the last vo- 

 lution and towards the aperture, leaving its inner margin a little elevated, and the 

 inner volutions a little below the plane of the last volution. The other specimen, 

 referred with doubt to this species, is distorted by pressure, and the upper volutions 

 of the spire are a little above the plane of the outer volution. The sinuosity in the 

 smaller specimen is wide and well defined : in the larger one it has, in the young 

 state, been similar to the other, but has become a narrow slit towards the aperture. 

 A similar change takes place in other species as they advance in age, till the notch 

 is finally entirely closed. 



Fig. 5, View looking upon the spire of a small specimen, showing the deep marginal sinuo- 

 sity and the strongly wrinkled surface. 



