BRACHIOPODA. 
169 
the upper and lower Pentamerus limestones the shell is rotund and the elongate 
form not represented. 
The absence of the specific type from the Oriskany fauna has yet to be 
accounted for. The normal Oriskany fauna of eastern New York is local, and 
the immigration of this species was probably excluded by the coarse, sandy 
character of the sediments, and their accompanying physical conditions. Where 
the fauna of the Oriskany is commingled with that of the Upper Helderberg, 
as in the arenaceous limestones of the Province of Ontario, Atrypa reticularis 
reappears with its Devonian aspect. 
In the Schoharie grit the expression of this shell is rendered peculiar by a 
flattening or sharp definition of the usually undefined fold upon the gibbous 
brachial valve. This peculiarity of the brachial valve is lost in the succeed¬ 
ing fauna (Corniferous limestone). Here we meet two distinct variants; (a) a 
small, elongate shell, like that common in the Lower Helderberg fauna, but 
invariably of less size (the A. ellipsuidea, Nettelroth); these are locally found in 
great numbers, indicating a gregarious habit; {b) a much larger, highly convex 
shell, having an outline intermediate between the others, and without the 
highly developed sinus of the pedicle-valve. This shell abounds throughout 
New York, though its occurrences are mostly in scattered or isolated areas. 
Passing to the Hamilton fauna, the prevailing forms are of medium size, with 
straight, somewhat extended cardinal line, moderately gibbous brachial valve 
and highly lamellose surface about the margins. These are accompanied rather 
sparingly by shells of great size, which do not, however, materially modify their 
external expression. In the calcareous beds of the upper Devonian, as in Iowa, 
these large shells become predominant, retaining the outline of their predeces¬ 
sors in the Hamilton group, but farther characterized by the lateral compression 
of the brachial valve. The smaller form, which occurs sparingly in the Che¬ 
mung sandstones of New York, is still similar to that prevailing in the Hamilton 
shales. The figures given by Professor Herrick,"^ of the shell occurring in a 
Devonian facies of the Waverly, or earliest Carboniferous fauna of Ohio, indi- 
* Herrick, Bulletin Scientific Laboratories of Denison University, vol. iii, j). 98, pi. iii, fig. 11, 1887 ; 
vol. iv, pi. ix, fig. 7, 1888 The expression of this Waverly shell, judging from the figures cited, is more that 
of the medium sized individuals of the Hamilton group than of the large forms of the later Devonian. 
