LOWER HELDERBERG ROCKS. 233 



Rliynclioiiella? bialveata (-n.s.)- 



Plate XXXIV. Fio. 1-4. 



Shell small, triangular or triangular-ovate, sometimes compressed. Valves 

 nearly equally convex : beak of dorsal valve incurved : beak of ventral 

 valve almost straight and subangular ; foramen narrow triangular, and 

 continued to the apex of the beak. 



Surface ornamented by from twelve to fourteen simple angular plications 

 on each valve ; the two central of which, on the dorsal valve, die out 

 a little before reaching the beak, near which they are somewhat de- 

 pressed, but towards the front they become slightly elevated above the 

 others, so as to form an indistinct mesial prominence. The middle pli- 

 cation on the ventral valve is smaller than the others, and depressed 

 near the front so as to produce a faint sinus, which extends about two- 

 thirds of the way to the beak, at which point the valve is most convex. 

 The two plications bordering the sinus are larger and more prominent 

 than those on each side of them, and become obsolete before reaching 

 the beak. A few faint imbricating lines of growth are visible near the 

 junction of the valves in front. 



Fig. 1 a, h. Dorsal and ventral views of a small individual. 



Fig. 2 a, b, c. Dorsal, ventral, and profile views of a less elongate form. 



Fig. 3 a, b, c. An individual having the plications nearly equal, and showing scarcely any 

 indication of a sinus in front. 



Fig. 4. An enlarged figure of the same species. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 



group : Albany county. 



Rliynclionella inutilis (n.s.). 



Plate XXXIV. Fio. 7 & 8. 



Shell subtriangular or subglobose. Beak of ventral valve small, closely 

 curved upon the opposite. Dorsal valve a little larger : beak incurved. 



Surface ornamented by eighteen or nineteen simple sharply elevated 

 plications, about four or five of which are elevated on the dorsal valve 

 so as to form a more or less distinct mesial fold, which' extends to about 



f PALiBONTOLOGY III.] SO 



