LOWER HELDERBERG ROCKS. 333 



becomes finely and obscurely plicate towards the aperture, and the left 

 side marked with two or three coarse plications, while the transverse 

 stria3 become more conspicuous and the longitudinal strias more subdued 

 or obsolete. 



This species bears some resemblance to P. ventricosvm ; bxit the last volution 

 expands laterally much more rapidly, and is never so ventricose as in that species; 

 while at the same time the obtusely angular form of the young shell, and the 

 plicate margins of the older ones, sufficiently distinguish this species. 



Fig. 5 a, b. Young specimens of this species. 



Fig. 5 c. A larger specimen, showing a shallow sinus on the back and the commencement 

 01 small plications ou the right side. 



Fig. 5 rf. A larger specimen where the plications are more fully developed. 



Fig. 5 e. Dorsal view of a similar specimen in which the plications are more developed, and 

 the obtuse carina extending from the apex is more defined, and ending in a broad 

 extension in front (which is broken off in the specimen figured ). 



Fig. b f. View of the aperture of the same specimen, showing the incipient plications upon 

 the left side of the aperture. This feature is much more strongly developed in 

 some specimens than in this one. 



Fig. 5 g. A specimen in which the spiral carina is more strongly developed, and is marked 

 by a deep sinuosity in the peristome. 



Fig. 5 k. An enlargement of the surface, showing the bending of the striaa as they cross the 

 carina. 



Fig. 5 i. An enlargement of the surface of a young specimen, where the longitudinal striae 



are more conspicuous. 

 Fig. 6 e. This 8p)ecimen is probably one of the same species, which is less expanded towards 



the aperture, and has one more volution at the apex than the preceding figures. 



Geological position and locality. In the middle portion of the shaly limestone : 

 Base of Helderberg mountains, Albany county. 



Platyceras gibbosum ( n. s.). 



^ Plate LIX. Fio.6 d - g, & 7 a. 



Shell obliquely subovoid, with one or two closely contiguous volutions at 

 the apex, from which the last one expands more or less rapidly, be- 

 coming ventricose in the middle and below ; upper part of the last 

 volution rounded, becoming angular and plicate below : aperture some- 

 what rounded ; peristome sinuous. 



