LOWER HELDERBERG ROCKS. ^ 319 



Platyceras pentalobus ( n. s.). 



Plate LVIII. Fig. 7 a, b, c. 

 Shell obliquely subconical : spiral with about two volutions ; apex and 

 upper volution smooth and rounded, becoming plicate on the last volu- 

 tion ; plications four, five or more ; peristome simple. 

 Sdeface marked by fine transverse stria?, which are strongly undulated 

 in passing over the plications. 



I have not been able to trace this species through its gradations of form ; but it is 

 so different from the other species observed, that I can have no doubt of its specific 

 distinctness. 



Fig. 7 a. View of the spire and the plications of the surface. 

 Fig. 7 b. View of the aperture and lower part of the spire. 

 Fig. 7 c. Profile or dorsal view. 

 Geological position and locality. In the central portion of the Lower Helderherg 

 group : Base of the Helderherg mountains, Albany county. 



Plat}ceras luultisiiiuatuui ( n. s.). 



Plate LVIII. Fia. 8 a, 6, c, & 9 a, b. 



Shell subdiscoid in the young state ; apex nearly on a plane with the 

 outer volution : volutions about three, the first ones minute; outer 

 one becoming free, ventricose, rounded or scarcely angular below, 

 somewhat flattened on the upper side, marked by several ridges and 

 shallow depressions on the upper and dorsal side : aperture somewhat 

 longitudinally oval ; peristome sinuate, with a deeper sinuosity on the 

 anterior margin. 



Surface marked by fine transverse striaB, which are strongly undulated 

 on the inequalities of the shell, and crossed by fine longitudinal or 

 revolving striae. 

 This is a^very distinct and pretty species, which, in its younger condition, shows 



all the volutions contiguous; but as it advances, the outer one continues in a direct 



line, and, expanding more rapidly, the shell loses its subdiscoid character as in fig. 



9 o. The plications become more strongly developed and more numerous as the shell 



grows older : at the same time, by the increase of the outer volution, the apex of 



the spire becomes depressed below its plane. 



