316 PALiEONTOLOGY OF NEW-YORK. 



Platyceras trilobatuiu ( n. s.)- 



Plate LVII. Fio. b a, b, c • 



Body of the shell obliquely or arcuately ovoid, trilobate : volutions three 

 or four, the last one (or more) becoming free, gradually expanding to 

 the aperture ; the apex closely involved and rising above the plane of 

 the outer volution, or sometimes on the same plane, concave towards 

 the suture : aperture subangularly ovate, sinuate on the right and left 

 sides, and the shell extended in front. 



Surface marked by two strong spiral depressions corresponding to the 

 sinuosities of the aperture, and crossed bylamellose striaB which are 

 strongly undulated on the sinuosities of the last volution, and are 

 marked by other undulations on the earlier volutions, indicating former 

 sinuosities in the margin of the aperture. 



The characters here given are well marked in several individuals, presenting a 

 contracted trilobate aspect, from the two sinuosities which extend in strong de- 

 pressions or grooves from the aperture to beyond the commencement of the first 

 volution. 



Fig. 5 a. View of the aperture apd part of the spire of a large individual. [ The striae re- 

 presented between the aperture and the next volution do not exist in the specimen.] 



Fig. 5 b. The lower or umbilical side of the same specimen, showing the sinus on that side. 



Fig. 5 c. The upper side of the specimen, showing the sinuosity on that side and the ex- 

 tension of the shell in the middle. The upper volutions are imperfect. 



Geological position and locality. In the upper part of the shaly limestone of the 

 Lower Helderberg group : Becraft's mountain and Schoharie. 



For descriptions of figures 6 a and 6 6 of Plate lvii, see under Pleurotomaria. 



Platyceras nnisiilcatnni ( n. s.). 



Shell depressed-globose or subdiscoid. Spire nearly on the same plane : 

 volutions about three, somewhat gradually expanding, the last one 

 ventricose, somewhat flattened on the dorsal side ; aperture campanu- 

 late, entire on the right side and sinuate on the left side. 



