290 
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 strim and a few distant wrinkles or undulations. Anterior wing 
marked by concentric strim and a few indistinct radiating striee; po¬ 
sterior wing, in both valves, with fine equal radiating strise. 
This is a small pretty species of which we have, in a single individual, the two 
valves in connexion presenting the unequal convexity of ordinary Aviculre. 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 the two valves connected at the hinge. 
Fig. 9. The 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. 
Avicula secmlformis (n. s.). 
Plate LIII. Fig. 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 costne and strong elevated 
concentric strke. 
The specimens figured have the shell partially or entirely removed, and the radii 
are not so strong as the perfect specimens would show. 
