162 PALiBONTOLOOT OF NEW-YORK. 



equally convex, and marked alike in perfect and unworn specimens. There is often a slight 

 difference in the form of the anterior wing, as will be seen on reference to the figures ; and 

 the posterior extremity is often obtusely angular, and sometimes rounded, depending on 

 the oblique trimcation. 



This shell approximates very nearly to the Modiolopsis arcuatus (PI. XXXV. fig. 8); 

 but the posterior extremity in that is broadly rounded, and there is no evidence of a posterior 

 wing. The one under consideration presents obscure evidences of a posterior wing in the 

 marginal expansion of the shell ; and this, with the existence of diverging radii, remove 

 it from the preceding genus. 



Fig. I a. A worn specimen, showing only obscure traces of radii, with strong concentric ridges. (Thif 



figure is taken from the original of A. aviformis.) 

 Fig I 6. A similar specimen, with obscure radii, liaving the anterior wing more extended. 

 Fig. 1 e. The opposite valve of a specimen of similar form as the last, with the radii better preserved. 

 Fig. 1 <1 A large specimen of the left valve, with strong radii. 



The presence or absence of the radii is here clearly shown to be due to wearing of the 



surface. 



Position and locality. In the compact central and higher portions of the Trenton limestone 

 at Middleville, and in the calcareous layers in the Utica slate at Coldspring, Montgomery 

 county. (Statt Collection.) 



211. 2. AVIGUL A ELLIPTIC A ( n. ip.). 



Pi.. XXXVI. Fig. 3. 

 Compare Jlvieula obliqua, Sowerbt, Sil. System, pag. 635, pi. 20, fig. 4. 



Inequivalve, subelliptical, narrowing towards the anterior extremity, compressed ; 

 umbones small, their direction nearly rectangular to the hinge line ; posterior wing tri- 

 angular, distinct above, and gradually merging into the margin of the shell ; anterior 

 margin sloping from the beak with a gentle curve, without a well defined wing ; cast of 

 the muscular impression in the left valve oblong, rounded above, and somewhat diverging 

 and striated below ; surface marked by a few obsolete concentric lines. 



This specimen has the greater part of the shell removed, and perfect specimens may 

 possess characters not obvious in this one. It corresponds in many respects with the one 

 cited above, and its similar geological position induces mc to regard it as an analogue at 

 least. 



It is doubtful if this species belongs to the true Avicula, and it may be found to approach 

 more nearly to the foilov/ing genus, which is distinguished from the Avicula by some 

 characters possessed by this. 



Position and locality. In the fine-grained black limestone, central part of the Trenton 

 limestone at Middleville. A rare shell. 



