463 
PALEONTOLOGY OF NEW-YORK. 
process a little below the apex, beneath which a foramen passes from the 
cavity of the valve, opening at the slightly elevated beak in an oval 
aperture. The hinge-plates are extended below along the surface of the 
shell, in strong ridges, which are dichotomized at the beginning of the 
muscular impression. Anteriorly these plates project in strong crural 
processes, which are straight only for a short distance, and then diverge 
in two slender points to the cavity of the ventral valve, in their main 
direction trending upwards into the dorsal cavity, and then by an abrupt 
geniculation proceed in a converging direction to the commencement of 
the crural plate. This plate is extremely elongate and very slender, deep¬ 
ly emarginate behind, with a slender process proceeding from the centre, 
which, with the two posterior branches, converge towards the bottom of 
the ventral cavity. 
This shell resembles very closely the R. ovoides , so abundant in the Oriskany 
sandstone in New-York, and it is possible that it maybe only a variety of that shell. 
Nevertheless all the specimens seen preserve the characters described, without any 
indication of gradation to the other form, which likewise occurs, in its large size 
and more coarsely striated surface, in the sandstone with this one. The details of 
hinge structure a very clearly shown in several individuals, and the elongate crural 
plate in a single specimen. 
Pig. 3 a. Dorsal view of a specimen of this species. 
Fig. 3 b. Ventral view of the same. 
Fig. 8 c. Profile or lateral view of the same. 
Fig. 3 d. Cardinal view of the same, having the ventral side upwards. 
Fig. 3 e. Front view of another specimen ( not well shown in the figure). 
Fig. 3/. The apex of the ventral valve (somewhat enlarged), showing the foramen, and the 
deltidial pieces forming the lower side of the foramen (the outer margins are too 
thick in the figure). 
Fig. 3 g. The interior of the ventral valve, showing the teeth, the dental plates, and mu¬ 
scular impression. 
Fig. 3 h. A similar specimen where the cavity of the valve is not so deep, showing the form 
of the muscular area and the places of the adductor muscles. The inner edges of 
the dental plates are seen extending into the cavity of the foramen. 
Fig. 3 i. The rostral portion of the two valves joined at the hinge ; the dorsal valve is 
downwards, showing the bases of the crural plates and the ridges proceeding from 
these. The opening of the foramen between the bases of the crural processes is 
very perceptible. 
Fig. 8 k. The interior of the upper part of a dorsal valve, showing the broad hinge-plates, 
the bases of the crura, the elevated median process, and the foramen below tho 
apex. 
