LOWER HELDERBERG ROCKS. . 317 



Surface marked by lamellose concentric striae, which are broadly undu- 



- lated and bending backwards on the upper side of the volution and 



forwards on the middle or dorsal side, while they are bent abruptly 



backwards on the lower side, and the line marked by a strong groove. 



This species resembles the P. gebhardi, but is hirger than that species usually 

 occurs, is flattened upon the back and canaliculate beneath; characters not observed 

 in that species. Diameter from anterior of mouth across the spire, nearly two and a 

 half inches. 



Geological position and locality. In the compact layers of the upper part of the 

 shaly limestone : Becraft's mountain. 



Platjceras teiiuiliratuiii ( n. s.). 



Platk LVIII. Fio. 1 - 5 ; and Plate LIX. Fig. 6 a, b. 



Shell small, depressed, subovoid or subdiscoid, with the last volution very 

 ventricose. Spire slightly raised above the plane of the outer volutions : 

 volutions about three, the first and second very minute, and the last 

 one rapidly expanding ; aperture campanulate, with the lip broadly 

 reflexed on the posterior and part of the right side. 



Surface marked by fine threadlike striae, which often become lamellose 

 on the last volution, and these are cancellated by very fine revolving 

 striae. 



The direction of the striae show that, in the earlier stages of growth, there was a 

 sinus near the centre of the anterior side of the aperture. In the progress of growth 

 this has been filled up, and another commenced nearer the spire, and also one on 

 the left side near the umbilicus, both of which are continued to the maturity of the 

 shell. The lower sinus is sometimes developed at an early stage of growth. 



PLATE LVIII. 

 Fig. 1. A young shell where the second sinus is not developed. 

 Fig. 2 a. A specimen where the second sinus has begun to be developed. 

 Fig. 2 b. View of the aperture and umbilicus of the same. The expanded peristome is broken 



off, except a small portion adjacent to the volution. 

 Fig. 3 a. Profile of a specimen which is more coarsely striated than usual. 

 Fig. 3 6. Spire of the same. 



