ORISKANY SANDSTONE. 488 



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. Valves 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 forma 

 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, oi* 

 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 wliat are termed the crural processes in Rhynchoneila, and 

 do not differ materially from the corresponding parts of Rhynchonella paittacea, 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 

 U deeply bifurcate, and grasps the medio-longitudinal septum of the ventral valve, giving additional strength 

 to the hinge. The ultimate extension and form of tliese 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, bad they been readily separable, like many others. 

 [ PALiEONTOLOGY III.] 55 



