380 
PALEONTOLOGY OF NEW-YORK. 
inclined to refer it to the Genus Megambonia; but have been unable to obtain 
sufficient evidence to determine this point. 
Professor M‘Coy refers this species of Conrad to the Genus Pterinea, regarding 
it as only a variety of the Avicula ( Pterinea ) retroflexa of Wahlenberg. The con¬ 
centric striae or lamellose elevations are never so strong or so regular in the Ame¬ 
rican species as in the Swedish specimens of A. retroflexa ; and since we do not yet 
know the hinge structure of the A. naviformis , I leave it as described by Mr. 
Conrad, under the Genus Avicula. 
Fig. 9 a. A left valve pi'eserving the entire form, with some remains of the shell, showing 
the concentric lamellose markings and the radiating striae. 
Fig. 9 c. A larger individual in which the radiating striae are scarcely shown. 
Fig. 9 b. A large individual, in which the shell is better preserved than in the other speci¬ 
mens : the umbo is proportionally less elevated, and the continuity of the hinge 
line is interrupted or obscured by the surrounding stone. The radiating striae are 
a little too strong in the figure. 
Geological position and locality. In the pentamerus limestone of the Lower Hel- 
derberg group : Schoharie, and Helderberg mountains. 
Avicula obscura (n. s.). 
Plate XLIX. Fig. 6. 
Shell subequilateral, circular below and on the sides, depressed-convex 
in the middle ; anterior wing narrow and short; posterior wing wider, 
and extending as far or farther than the posterior extremity of the valve 
(imperfect in the specimen). 
Surface marked by unequally dichotomizing radii, which are cancellated 
by concentric laminse, the latter becoming much stronger on the lower 
part of the shell. 
The specimen figured is the interior of a right valve, having the anterior wing 
nearly obliterated, and the other broken off at some distance from the extremity. 
It appears to have been a true Avicula , and is the only one observed in the Ten- 
taculite limestone. 
Geological position and locality. In the Tentaculite limestone : Schoharie county. 
