244 PALEOZOIC PALEONTOLOGY. 
The dimensions of the largest and best specimen observed are: 
length, 45 mm., and height, 28 mm. 
Remarks.—Hall’s original figures of Avicula subplana allow a great 
deal of yariation in the outline of the species. The several Decker 
Ferry specimens referred here also exhibit considerable variation, but 
none of them correspond exactly with either of Hall’s figures, the 
posterior margin being more sinuate, with the posterior extremity of 
the hinge-line more acute. The species is here referred to the genus 
Pteronites, as it seems to be more nearly in accord with members of 
that genus than with any other Paleozoic genus, although in no speci- 
mens have the hinge characters been preserved, nor has the right valve 
been seen. Heretofore the members of this genus have not been re- 
corded earlier than the Chemung fauna of the Devonian, and all of the 
deseribed species are from the Chemung and the lower Carboniferous 
faunas. | 
PTYCHOPTERIA ? SUBQUADRATA Nh. Sp. 
Plate XXII., Fig. 2. 
Description.—Right valye unknown. Left valve subquadrangular 
in outline, depressed-convex, with anterior and posterior wings; hinge- 
line a little shorter than the total length of the shell, the beak pro- 
jecting slightly beyond the hinge-line and situated about one-third 
the length of the shell from the anterior margin. The anterior 
margin forms nearly a right angle with the hinge-line; below it rounds 
regularly into the arcuate ventral margin; posteriorly the ventral 
margin joins the posterior margin with a rather abrupt turn; the 
posterior margin is gently convex and joins the hinge-line in an obtuse 
angle. The surface of the shell is marked by fine, concentric lines 
of growth. 
The dimensions of the type specimen are: length, 30 mm.; length 
of hinge-line, 25 mm.; height at beak, 23 mm. 
Remarks.—The hinge characters of this shell are unknown, but in 
general form and markings the specimens resemble members of the 
genus Ptychopteria more closely than any other Paleozoic genus, and 
it is therefore placed here provisionally, although that genus has not 
hitherto been recognized earlier than the Chemung fauna of the De- 
vonian. ‘This species differs from any of the other members of the 
genus in its less obliquity and in its more nearly vertical anterior 
margin. 
