CONTRIBUTIONS TO TALuEONTOLOGY. 35 



PLx\TYCERAS ETCTUM ( n.s.). 



Shell very depressed and obliquely subconical, the width equalling 

 or exceeding the height. Apex minute and enrolled to about one 

 and a half turns, when it abruptly expands, spreading more upon 

 the right posterior side and becoming elongated in front, giving 

 a peculiar oblique foi-m to the shell. Aperture obliquely ovate : 

 peristome entire, or with a slight sinuosity on the left posterior 

 side; posterior side not contiguous to the preceding volutions. 



Surface marked by fine undulating concentric striae : a few broad 

 unduLitions mark the surface longitudinally. Width of aperture 

 about an inch and a quarter, and a little greater from the anterior 

 to tlie posterior margin : height about an inch. 

 This species is very expanded, with surface striae differing from any of 



the preceding. 



Geological formation and locality. In shales of the Hamilton group : 



Ontario county, N.Y. 



PLATYCERAS CYMBIUM ( n. s.). 



Shell subangularly ovoid. Apex minute, making one or more turns, 

 below which the shell is abruptly expanded; the back flattened 

 or but moderately convex, while the right side is distinctly flat- 

 tened, making an obtusely angular ridge between the side and 

 back of the shell, and sloping more gently on the left side. 

 Aperture extremely spreading. 



Surface marked by transverse strise, which are undulated on the 

 dorso-lateral angle. Shell lamellose striate, with a few strong 

 nodes or spines : these, in the specimens examined, are broken 

 so that their extent is unknown. 



This species is characterized by the broad expansion of the aperture, the 

 flattened dorsum and right or upper side, giving an obtuse dorso-lateral 

 angle. 



Geological formation and locality. In Upper Helderberg limestone, at 

 Clarence-hollow, N.Y. 



PLATYCERAS FORNICATUM ( n. s.). 



Shell obliquely subhemispherical, or very depressed obliquely 

 conical. Apex minute, distinctly spirally enrolled for about one 

 turn and a half, below which it expands, so that in the extent of 

 an inch and a half along the dorsum to the front it has acquired 



1861.] 



