S8« PAL-EONTOLOGY OF NEW-YORK. 



Avicula coiiiiiiuiiis ( n. s.)- 



Plate LII. Fio. 1 - 7 ; and Plate LIII. 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) j 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 LIT. 

 Fig. 1. A cast of the left valve which has been transversely compressed, elevating the 

 anterior and dcpresBing the posterior wing. The radii are pretty regularly inter- 

 rupted by the concentric striae. 



