NIAGARA LIMESTONE OF CHICAGO, ILLINOIS. 95 



Chesney, but our species has two additional ribs on each side of the ventral sinus. We 

 have not access to Hall's description of P. ? ventrlcosns from the Niagara limestone of Wis- 

 consin. 



Rhi/nchonella neglecta Hall. Pal. N. Y., II. 274, pi. lvii. fig. 1, a— p. 



Pterinea (Goldf.) Meek. 

 Pterinea volans W. and M. 



Shell large, very oblique, with an extended hinge-line, and alate posterior extremity. 

 General outline nearly semicircular. Left valve moderately inflated anteriorly, flattened in 

 the posterior half ; beak small, nearly terminal, incurved, not surpassing the hinge-line ; 

 whole pallial outline regularly circular, except a rather deep sinus beneath the posterior 

 hinge extremity. Cartilage facet not certainly determined ; in one small specimen it ap- 

 pears to be very wide, with four or five furrows. External surface marked by strong ribs 

 which bifurcate and become wavy beyond the middle in old shells ; the ribs becoming 

 abruptly much smaller on the posterior wing. Besides the ribs, the superficial layer of the 

 shell is cancellated by strong concentric and feeble radiating ridges. The costaj show dis- 

 tinctly on the cast, except on the posterior wing. 



Length of hinge-line, 2.1 inches ; greatest height of shell (at middle of hinge-line,) 1.5 

 inch ; convexity of left valve, .26 inch. 



This species differs from P. stricecosta, in its less rugose reticulations, stronger ribs, and 

 straight, instead of curved, umbonal slope. Young specimens resemble P. (Avicula) emacerata 

 Hall, but the posterior wing is more expanded, so as to constitute half the surface of 

 the valve. 



Pterinea revoluta W. and M. 



Plate II. figure 12. 



A distinct and well-marked species known only by the impression of the left valve, which 

 is beautifully restored by means of a c/idta percha cast. This is rotund-quadrate in outline, 

 ventricose, with depressed beak, hinge-line but slightly produced posteriorly. Anterior 

 margin regularly rounded to the ventral side ; posterior slightly sinuate below the hinge 

 extremity ; greatest dimension from the hinge to the circularly curved ventral side. The 

 surface is marked by seven or eight concentric, lamellose folds or layers of substance, each 

 of which is ornamented by a distinct set of numerous rounded stria). These are not con- 

 tinuous from fold to fold ; neither are they straight, nor strictly radiating from the beak ; 

 but they generally exhibit, especially toward their upper ends, a convexity toward the pos- 

 terior side ; in other words, the upper ends of the strise on each concentric fold are bent 

 forward. The margin of the valve is strongly revolute. 



Length from beak to ventral side, .56 inch ; length along hinge-line, about .47 inch. 



This curious and pretty species recalls P. planulata Conrad, and P. Mam McCoy, from the 

 Amesbury limestone, but the description given above distinguishes it decisively from these 

 and all others known to us. 



Pterinea cyrtodontoid.es W. and M. 



Shell of moderate size, oblique ; hinge-line equal to greatest width ; its extremities 

 rounded, scarcely alate ; beaks anterior to the middle. Left valve ventricose, with an in- 



