PALAEONTOLOGY OF NEW-YOHK. 
Avicula communis (n. s.). 
Plate LII. Fig. 1 - 7 ; and Plate LIIL Fig. 1, 4 & 6. 
Shell obliquely ovate; the left valve gently convex in the middle, and 
becoming gibbous towards the beak, which in the young shell is narrow 
and projecting above the hinge-line : right valve flat or gently concave 
in the middle and below, and becoming slightly convex on the umbo; 
anterior side gently curving to the base which is broadly rounded, the 
curvature of the posterior side being more abrupt : anterior wing small, 
trigonal, obtuse at its extremity, strongly defined from the body of the 
shell; posterior wing three times as long as the anterior wing, obtusely 
or subacutely pointed, extending more or less beyond the margin of 
the shell, concave on the outer or lateral margin, its junction with the 
body of the shell not strongly defined. 
Surface of left valve marked by slender, sharply defined, rounded radii, 
the principal of which are distant from two to four or five times their 
width, and the spaces occupied by one, two or three finer interstitial 
radiating striae (these radii are but faintly, and sometimes not at all 
perceptible on the posterior wing, except along its upper margin, while 
they are not seen on the anterior wing); concentrically marked by fine 
lamellose striae, which, in the more perfectly preserved surfaces, are 
elevated and subimbricating : these striae are usually conspicuous on 
both the anterior and posterior wings. Surface of the right valve marked 
by broader and scarcely elevated radii and less defined concentric striae. 
This species is the most common form of Avicula in the shaly limestone, or indeed 
in any part of the Lower Helderberg group. In its different stages of development, 
and different degrees of preservation, it presents considerable variety of aspect and 
surface marking. In many of the casts the stronger radii are interrupted, and fre¬ 
quently with great regularity, by the concentric laminae, which leave depressions 
cutting the radii : others are less regularly interrupted. 
PLATE LII. 
Fig. 1. A cast of the left valve which has been transversely compressed, elevating the 
anterior and depressing the posterior wing. The radii are pretty regularly inter¬ 
rupted by the concentric striae. 
