LOWER HELDERBERG ROCKS. 389 



PLATE LII. 



Fig. 9. The left valve of this species, preserving the posterior wing and the greater part of 

 the body of the shell, the anterior portion being broken off. 



Fig. 10. The interior of a left valve, which, in the general form and obliquity of the body 

 of the shell, corresponds to this species, but the wing is shorter. The abrupt 

 separation between the body of the shell and the wing may be due to pressure, 

 which has produced a slight folding along that line. 



PLATE LIII. 



Fig. 2. The right valve of a young shell which is imperfect, but, in the obliquity of the body 

 and general form, corresponds to this species. It is moderately convex, a little 

 gibbous on the umbo, and the surface of the body of the shell shows subdued 

 radii, while the wing shows only concentric markings as in A. communis. 



Fig. 3. The left valve of a young shell preserving the surface markings, with radiating striaa 

 upon the wing, as in the larger specimens. The posterior part of the wing is 

 broken off, so that the full extent is not shown in the figure. 



Fig. 5. The interior of the left valve of a shell of this species. In the figure, the sinus on 

 the outer magin is too abrupt : a cast from this interior presents precisely the 

 characters shown in fig. 9, Plate lii. 



Fig. 7. The left valve of a small specimen of this species : the anterior wing and beak are 

 broken off, as well as a part of the posterior wing. 



Fig. 10. A cast of the left valve of this species, from which the shell has been exfoliated 

 and the posterior and basal margins broken off. The specimen is a little more 

 abruptly convex in the middle and upper part than usual in shells of this species. 



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

 group : Albany and Schoharie counties; and Litchfield, Herkimer county. 



Avicula bellula ( n. s.). 



Plate LIII. Fig. 8 & 9. 



Shell subrhomboidal, with a rotund base ; length equal to once and a 

 half the height ; hinge-line a little shorter than the length of the shell : 

 left valve convex, or a little gibbous in the middle ; right valve nearly 

 flat or very slightly convex : anterior margins abruptly rounded ; po- 

 sterior margin a little produced and rounded, giving a broad regularly 

 curving ventral margin : anterior wing small, triangular, acute, pro- 

 jecting as far as, or a little beyond the margin of the shell, distinctly 

 separated by a sinus from the body of the shell ; posterior wing 

 broader, not conspicuously separated from the shell, nor reaching as 

 far as the posterior margin. 



Surface of the left valve marked by strong dichotomizing or somewhat 

 [ Paljkontoloot III.] 87 



