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 Atrijpa reticularis, occurring in the Niagara fauna at 

 Waldron, Indiana, the free concentric lamellte 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 

 Atrypa 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 Vebnbdil. Geologie de la Russie, etc., p. 93, pi. xi, tig-. 13. 1845. 

 ScHNDR. PalsEoiitographica, vol. ili, p. 181, pi. xxiv, fig. 4. 1854. 

 Kayskr. Abhanill. fJeol. Specialkarte von Pi'euss. u. den Thiir. Staat., pp. 184, ISfi, pi. xxviii, 



figs. 4-6. 1878. 

 TscHERNYSCHKW. Fauna des unt. Devon am West-Abhange des Urals, p. 42. 1885. 



