4«JI 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 may be 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. 



Fig. 3 a. Dorsal view of a specimen of this species. 



Fig. 3 6. Ventral view of the same. 



Fig. 3 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 

 dcUidial pieces forming the lower side of the foramen ( the outer margins are too 

 thick ia 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 »'. The rostral portion of the two valves joined at the hinge : the dorsal valve is 

 downwards, showing the hases 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 the 

 aficx. 



