of the Devonian of Western Tennessee. 737 



ista tennesseensis, Meristina roemeri, Phacops sp., and 

 six species of gastropods. 



The presence here of Dictyonella subgibbosa n. sp., 

 Nacleospira concentrica, and Merista tennesseensis at 

 first inclined the writer to refer this formation to the 

 Silurian, On the other hand, the preponderance of the 

 fauna is distinctly Devonian and the general paleogeo- 

 graphic conditions likewise strongly support this refer- 

 ence. The Upper Silurian in America was a time of 

 marked restriction of the seas, scarcely a normal marine 

 fauna being known anywhere of Upper Silurian time. 

 Furthermore, no deposits whatever of this age have yet 

 been found anywhere in the whole Mississippi basin. 

 The Rockhouse fauna, however, is a normal marine 

 assemblage, and is present in both Tennessee and Okla- 

 homa. It represents, therefore, a wide embayment in 

 this region, which is known to have been an early Devo- 

 nian basin. The preserval of so thin and soft a forma- 

 tion in both Tennessee and Oklahoma beneath Helder- 

 bergian strata further supports the belief that it was not 

 subjected to long erosion before the deposition of the 

 latter. 



The Devonian age of the fauna is indicated by the 

 deployment of the gastropods, which make up one fourth 

 of its number, for while gastropods are commonly abun- 

 dant in the Lower Devonian, the New Scotland having 

 thirteen and the Oriskany over twenty species, on the 

 other hand but two or three species are found together 

 in any Silurian fauna. Likewise the abundance and 

 variety of orthoid brachiopods is a Devonian characteris- 

 tic. Neither Pleurodictyum nor Edriocrinus is known in 

 the Silurian, and the two species of the latter both charac- 

 terize the succeeding Linden formations. Large speci- 

 mens of Rhipidomella oblaia, like those of this formation, 

 are never seen in the Silurian, and the Delthyris is closely 

 related to perlamellosa, which is so typical of the Helder- 

 bergian, and is much larger and more coarsely lamellose 

 than- any Silurian species of this genus. 



Although this is the first known occurrence of a Dicty- 

 onella in the Devonian in America. Barrande described a 

 species long ago from the Middle Devonian (stage G) of 

 Bohemia. The presence of the Silurian holdovers, 

 Bilobites bilobus, Nucleospira concentrica, and Merista 

 tennesseensis, however, as well as the primitive expres- 



