No. 109.] 109 



dorsal valve flat ; beak straight : hinge sloping from the beaks 

 at an angle of 110° to 160°, rounded at the extremities. Surface 

 marked by twelve or thirteen simple angular plications ; two 

 of which, on the middle of the ventral valve, are a little larger 

 and slightly more prominent than the others : between these, 

 there is a third smaller depressed plication, forming an in- 

 distinct sinus. On the dorsal valve the two middle i)lications 

 are a little closer together, and slightly more prominent near 

 the front, tlian the others ; while the depressions separating them 

 from these, each side, are a little wider and deeper than those 

 between the other plications. 



This species presents some varieties of form, apparently due to age ; the 

 hinge-line of younger or smaller individuals being more extended and more 

 nearly straight. The silicified condition of the specimens has obscured the 

 finer surface marlvings. Many of the specimens appear to retam remains 

 of fine radiating stria) ; while more distinct concentric lines, and occasional 

 stronger undulations of growth, are visible in most of them. 



Along the hinge-line, in many specimens of this species, there is a pecu- 

 liar fimbriated appendage apparently coming from between the valves, and 

 anchylosing them firmly together. I have noticed this appendage in so many 

 instances, that I am inclined to believe that it had some connexion either 

 directly with the animal, or with the cardinal apparatus. 



Geological position and locality. Oriskany sandstone : New-York, 

 Maryland and Canada. 



Orthis ^QUIVALVIS. 

 Pal.N.Y. Vol.iv. 



Shell subcircular, compressed ; hinge about three-fourths the 

 width of the shell : dorsal valve depressed-convex : ventral 

 valve flattened, much depressed or concave towards the front, 

 slightly elevated in the umbonial region ; foramen of medium 

 size; area moderately high, extending to the extremities of the 

 hinge. Surface marked by numerous small radiating strife, which 

 bifurcate two or three times between the beak and the margins. 



Geological position and locality. Limestone of the Upper Helderberg, 

 Williamsville, N.Y. 



