172 
PALEONTOLOGY OF NEW YORK. 
during the epoch of the Oriskany sandstone, and that of the coarse-ribbed type 
broken by an hiatus extending from the close of the Niagara to the opening of 
Corniferous epoch, this is a purely local or American peculiarity. There is 
abundant evidence in the works of European writers, of the presence of both 
forms in faunas of Russia and Germany which are essentially homotaxic with 
the Lower Helderberg and Oriskany of this country."^ 
In the variant of Atrypa reticularis, occurring in the Niagara fauna at 
Waldron, Indiana, the free concentric lamellae frequently show a tendency 
to fold inward at the summit of the principal plications. The infolded 
edges fail to unite, and this tendency to the formation of tubules is apparently 
carried no further at this period. More extreme results were attained by the 
Atnjpa aspera of the Hamilton shales, or possibly by its migrated ancestor, dur¬ 
ing the period of time represented by the deposition of the Lower Helderberg, 
Oriskany and Upper Helderberg sediments. At all events, the Atrypa spinosa of 
the Hamilton shales is but an A. aspera with the lamellae enfolded into tubular 
spines. Intermediate stages connecting these different phases are not present 
in this fauna; it is furthermore evident that these spines are an early genetic 
condition, being found on the youngest portions of the adult shell; both of 
these facts pointing to the attainment of this condition at an earlier period. 
This spinose form is continued into the Chemung faunas (A. hystrix), with 
some modification of expression, the spines being few and long, and the 
plication of the surface very coarse and quite simple; the shell in its 
decline thus representing a decided return to the primitive type of 
structure. 
Contemporaneously with the form of A. reticularis in American faunas, 
appears another, the Atrypa marginalis, Dalman, which, according to Salter and 
Davidson, actually antedates A. reticularis in Great Britain, where it is stated to 
occur as low down as the Caradoc. 
* See d’Archiac and de Verneuil. Geologie de la Riissie, etc., p. 93, pi. xi, fig-. 13. I84.fi. 
ScHNCR. Pala3ontographica, vol. ill, p. 181, pi. xxiv, fig. 4. 18fi4. 
Kayser. Abhandl. Geol. Siiecialkarte von Preuss. u. den Thiir. Staat., pp. 184, 18fi, pi. xxviii, 
figs. 4-6. 1878. 
Tschernysciiew. Fauna des unt. Devon am West-Abliange des Urals, p. 42. 188fi. 
