aOO PALAEONTOLOGY OF NEW- YORK. 



fasciculate radii ; the right valve, by more numerous, equal and finer 

 radii : entire surface of both valves marked by equal, elevated con- 

 centric striae and a few distant wrinkles or undulations. Anterior wing 

 marked by concentric striae and a few indistinct radiating striae ; po- 

 sterior wing, in both valves, with fine equal radiating stria). 



This is a small pretty species of which we have, in a single individual, the two 

 valves in connexion presenting the unequal convexity of ordinaiy Aviculae. The 

 markings are only obtained from the cast of the interior, and from the interior of 

 separate valves, and are doubtless more strongly shown on the exterior of the shell 

 than represented in the figures. 



Fig. 8. A specimen showing tlie two valves connected at tiie Iiingc. 

 Fig. 9. Tlie interior of a left valve of the same species. 



Geological position and locality. In the shaly limestone of the Lower Helderberg 

 group : Helderberg mountains, and Schoharie. 



Avicala securifoniiis ( n. s.). 



Plate LIII. Fro. 11 - 14. 



Shell subrhomboid-ovate, slightly oblique ; length and height varying 

 from nearly equal, to the height one-fourth greater than the width, 

 moderately convex in the middle and on the umbo : anterior margin 

 long, slightly concave above and curving to the base ; posterior margin 

 below the sinus, somewhat abruptly curving into the broad rounded 

 basal margin : anterior wing small, trigonal, subacute, distinctly se- 

 parated by a sinus from the body of the shell ; posterior wing large, 

 subacute at the extremity, not strongly distinct from the body of the 

 shell, extending as far as or a little beyond the margin of the shell ; 

 marginal sinuosity long and shallow. 



Surface marked by moderately strong radiating costae and strong elevated 

 concentric striae. 



The specimens figured have the shell partially or entirely removed, and the radii 

 are not so strong as the perfect specimens would show. 



