LOWER HELDERBERG ROCKS. 
933 
Uliynclionella ? bialveata (n. s.). 
Plate XXXIY. Fig. 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, b. 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 study limestone of the Lower Heidelberg 
group : Albany county. 
Rhyncltonella inutilis (n. s.). 
Plate XXXIY. Fig. 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 
[ Palaeontology III.] 30 
