ORISKANY SANDSTONE. 
433 
The ventral valve is usually nearly flat or slightly convex near the 
beak, flattened or concave in the middle, with a broad deep sinus ex¬ 
tending thence to the front of the shell; the anterior extension being 
often turned at right angles to the plane of the longitudinal axis. Beak 
small, elevated, and closely incurved over the umbo of the opposite valve, 
perforate : no area. Dorsal valve convex, often ventricose, with a deep 
sinus in the anterior margin. Yalves articulating by teeth and sockets ; 
the anterior and antero-lateral margins often crenulate or plicate within. 
The cardino-lateral margins of the ventral valve are usually angularly 
inflected, and embraced within the edges of the dorsal valve. 
The valves articulate by means of two teeth in the ventral valve with 
corresponding sockets in the dorsal valve, and a medio-longitudinal ridge 
in the ventral valve which is more or less completely embraced between 
the deeply bifurcating cardinal process of the dorsal valve, which forms 
part of the apophysary system. 
The dorsal valve has a prominent bifurcating cardinal process, the 
branches of which, slightly diverging, form the first or lower crural 
processes, which are directed upwards and inwards, or, when the valves 
are closed, are directed into the muscular cavity of the ventral valve 
(their upper surfaces slightly grooved )*. Below these first processes, and 
proceeding from the origin of the thickened cardinal process, another 
pair of crura are directed inwards, and gently curve towards the first, 
the medio-longitudinal ridge being continued to the centre of the valve; 
and on each side, from these second crural processes, a ridge proceeds 
along the inside of the shell nearly parallel to the margin. This ridge, or 
thickening of the shell, marks the extent to which the margin of the 
ventral extends within the edge of the dorsal valve. 
* These processes correspond precisely with what are termed the crural processes in Rhynchonella, and 
do not differ materially from the corresponding pai’ts of Rhynchonella psittacea, except in their prominence 
or extension beyond the cavity of the shell as shown in the figures. In the older shells, at least, this process 
is deeply bifurcate, and grasps the medio-longitudinal septum of the ventral valve, giving additional strength 
to the hinge. The ultimate extension and form of these processes is still undetermined. Separate valves of 
the species of this genus are rarely observed even in positions where the entire shells are common, and 
where they would have been preserved, had they been readily separable, like many others. 
[ Paleontology III.] 55 
