No. 109.] 155 



the shell, angular at the extremities : dorsal valve convex, but 

 much more compressed than the opposite ; mesial fold narrow, 

 abruptly elevated, flattened or slightly grooved in the middle ; 

 beak unknown : ventral valve very prominent at the umbo, 

 from which it slopes abruptly to the margins ; mesial sinus 

 narrow, deeply impressed ; sides sloping ; base flat ; beak un- 

 known ; area high, triangular, slightly arcuate ; foramen nar- 

 row, finely striate longitudinally and transversely. Surface or- 

 namented by about eighteen to twenty simple, regular, rounded 

 plications on each side of the mesial fold and sinus, concentri- 

 cally marked by fine closely arranged lines of growth. 



The specimens of this species in my possession are somewhat imperfect; 

 a single individual with both valves attached has the .bealis imperfect. The 

 beali of the dorsal valve projected beyond the plane of the area of the 

 opposite valve : the high area of the ventral valve is slightly arched near 

 the beak. 



There is much general resemblance between this species and the S. 

 eurutines of Owen, but the beak of the ventral valve is more arcuate, 

 the plications finer, and the mesial sinus and fold more angular. 



Geological position and locality. In limestones of the age of the Ha- 

 milton group : Illinois. 



SpIRIFER FORNAX ( n. S.). 

 Pal. N.Y. Vol. iv. 



Shell semicircular, width a little more than twice the length ; 

 hinge equalling the greatest width of the shell, angular at the 

 extremities : dorsal valve depressed-convex, having a narrow 

 depressed rounded mesial fold, sloping from the beak very ab- 

 ruptly with a slight curve to the front and sides : ventral valve 

 having the sinus moderately broad, shallow and rounded ; beak 

 scarcely arched ; area high, slightly arcuate, and inclined a little 

 backwards over the hinge-line ; foramen somewhat narrow. 

 Surface marked by fifteen to eighteen simple rounded plications 

 on each side of the mesial fold and sinus. 



