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 Oriskanj 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 Univereity, vol. iii, p. 98, pi. iii, fig. 11, 1887 ; 

 vol. iv, pi. ix, fijf. 7, 1888 The expression of this Waverly shell, judging from the figures cited, is more that 

 of the mediom sized individuals of the Hamilton group than of the large forms of the later Devonian. 



