Report of the State Geologist. 



433 



belong to the genus MerUtella* in which the loop is produced at the 

 union of its arms into two circular branches, while in Meristina\ and 

 Whitfieldia of the Niagara, the loop terminates in a short, sharp, simple 

 process, or in a blunt, bifurcate extension. In this progressive com- 

 plication of the loop, the simplest forms are limited to the earlier 

 faunas. 



The development of Rensselceria appears to be of essentially Lower- 

 Devonian import. The genus attains its greatest development in the 

 Oriskany and appears only sparingly thereafter, while it is not known 

 to occur in the Niagara. Another Terebratuloid genus similar to, if 

 not identical with Cryptonella, is also known in the Lower Helderberg. 

 The development of Pentamerus in the American and European 

 pala30zoic is essentially different. For example, P. galeatus, which is 

 confined to the Lower Helderberg, is limited in England to the Upper- 

 Silurian, but in Germany passes upward into the lower Upper-Devo- 

 nian, subject to some, but unessential, variations. Again, the 

 genus, under its strict limitations, is, in America, almost exclusively 

 confined to Silurian faunas, while on the continent of Europe, 

 large, coarse-ribbed species are distributed throughout the Lower- 

 Devonian. The phenomenon is precisely the reverse of that pre- 

 sented by the genera Goniatites and Cryphceus, which, in the Con- 

 tinental Devonian put in an earlier appearance and attain a much 

 greater specific development than here. 



Anastrophia is known only in the Lower Helderberg and Niagara 

 faunas. 



Rhynchonella is extremely prolific in species, but can hardly be said 

 to present diagnostic features, save in the obese forms, R. mutabihs, 

 pyramidata, ventricosa. which have a Silurian expression. 



Eatonia is limited to the faunas of the Lower Helderberg and 

 Oriskany. 



Pteropoda. Conularia, usually a rare form in the Silurian, was 

 comparatively abundant (G. pyramidalis). 



Tentaculites is extremely abundant, often constituting entire layers 



♦That is. Meristella in its current acceptation, or with the internal structure as 

 in M. arcuata and as described by Prof. Hall in 1867 (Pal. N. Y. Vol. IV. p. 298). The 

 first type of the genus, the species for which the name was proposed in 1858 (12th 

 Rept. N. Y. State Cab. Nat. Hist.) is M. naviformis of the Clinton group, and the pre- 

 cise character of its loop is not yet understood. In the second use of the term (13th 

 Rept. State Mus., p. 73), where the genus is defined, it is evident that the group is based 

 on the Atrypa tumida Dalman., subsequently used by Davidson as the type of his genus 

 Whitfieldia. 



t Also in its current signification. The type of this genu3 was M. Maria, regarded by 

 Davidson, with undoubted accuracy, as identical with Dalman's Atrypa tumida, and 

 therefore referred to his Whitfieldia. It thus appears that Atrypa tumida has been taken 

 as the type of two, if not of three, distinct genera. 



55 



