BEEKMANTOWN AND CHAZY FORMATIONS OF CHAMPLAIN BASIN 523 



factors in the distribution of the family. Its more northern distribu- 

 tion is, in my opinion, the rather accidental result of its restriction to 

 certain marine basins, notably the Baltic basin, whence it reached, 

 in Lower Siluric time, the Newfoundland embayment along a route 

 at present not quite apparent. 



The absence of the Lituitidae from the Champlain and Mississip- 

 pian basins during Champlainic or Lower Siluric time is quite sug- 

 gestive as indicating a lack of free communication between the Baltic 

 and Mississippian basins in that era, such as is claimed by some 

 geologists, apparently on good grounds, for the Upper Siluric time 

 by way of the Arctic regions. 



If we sum up the evidence furnished by this analysis of the distri- 

 bution of the cephalopod genera, we find that one genus (Trocholito- 

 ceras) is restricted to the Champlain basin, one (Litoceras) to the 

 Newfoundland embayment, two (Nanno and Gonioceras) are found 

 only in the Champlain basin in Chazy time and persisted in the 

 Mississippian sea (in the case of Gonioceras in the Pacific-American 

 basin) ; further, that Tarphyceras, Deltoceras and Plectoceras 

 (and probably also Aphetoceras) are restricted to the Newfoundland 

 embayment and Champlain basin (extending in the two first named 

 into the central Appalachian trough) ; that Piloceras and 

 Eurystomites find their principal development in the Newfoundland 

 embayment and Champlain basin, but that while the former in one 

 species also reached the British embayment, the latter is entirely 

 restricted in Beekmantown and Chazy times to the Newfoundland 

 embayment, the Appalachian trough and Mississippian sea. On the 

 other hand, the Lituitidae flourished in the Baltic basin in Lower 

 Siluric time, reached the Newfoundland embayment with but one 

 or a few representatives, and are not known from either the Appa- 

 lachian trough or the Mississippian sea. The genera Schroedero- 

 ceras and Trocholites also attained plainly their maximal develop- 

 ment in the Baltic basin ; the former found its way into the Cham- 

 plain basin in Beekmantown (Fort Cassin) time with two species 

 and the latter with one, and the genus Barrandeoceras ex- 

 tended from the Bohemian-Mediterranean basin to the Newfound- 

 land embayment and Champlain basin. 



We have hence from their geographic distribution in Beekman- 

 town and Chazy times four larger groups of cephalopod genera : 



I Those which are known only in the Champlain basin and later 

 are also found in the Pacific- American basin : Nanno, Gonioceras, 

 Trocholitoceras. 



