504 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



history in these ages is practically unknown; and their Neopaleozoic de- 

 velopment in the Pacific realm likewise is obscure. But the Pennsyl- 

 vanian representatives of both classes in the Pacific realm are much like 

 those in the Atlantic. Of course there are many more or less peculiar 

 local developments, as in the Guadalupian of Texas, the Salt Eange in 

 India, the Urals in Eussia, and Sicily in the Mediterranean, but these 

 are connected by enough of cosmopolitan genera so that the general as- 

 pect of these faunas is not greatly unlike those prevailing during this 

 age in the Atlantic basin. 



The Crustacea, as is indicated by the great development of trilobites and 

 phyllopods in the Cambrian deposits of western North America, probably 

 originated in the Pacific realm long before the Eopaleozoic. The known 

 Cambrian phyllopods are easily separable into (1) the north middle 

 Atlantic and Baltic types, found in the northern Appalachian and the 

 Saint Lawrence troughs in America and the British Isles and the Swed- 

 ish Baltic region in Europe, and (2) the Pacific types found in the 

 Cordilleran region of western North America. However, several species 

 found in China are closely allied to Saint Lawrence forms. Eegarding 

 the trilobites, the Cambrian types divide up much the same as the phyl- 

 lopods in origin and distribution of genetically related species and genera. 

 Essentially the same geographic distribution of the trilobites continued 

 to the close of the Ordovician, except that the communication between 

 the Saint Lawrence, Baltic, and Arctic faunas seems to have grown more 

 intimate, and that at times the south Atlantic fauna mingled with that 

 of the Pacific or simply extended into areas commonly occupied by the 

 latter. In the Ozarkian, for instance, a distinct trilobite f acies, dis- 

 tinguished especially by DiJcelocephalus and believed to have been de- 

 veloped in the Gulf of Mexico and Caribbean basins, invaded the Ohioan 

 province to Quebec on the one hand, and by way of the lowan basin 

 and the Great Basin, to Nevada, on the other. The upper Cambrian. 

 (Saint Croix) invasion of this southern Atlantic fauna differed in that 

 it did not extend beyond the Virginias in the Cumberland basin and in 

 that it did extend southward (as well as northwestward) from north 

 Arkansas through Oklahoma and New Mexico to Arizona. Whether this 

 fauna in either case actually mingled with a true Pacific fauna in Nevada 

 seems questionable. Except in the Mississippi Valley proper and in 

 the southern Appalachian region, the Canadian deposits in America fre- 

 quently contain Asaphus-like tribolites. So far as known, this genus does 

 not occur in the succeeding Ordovician formations in America, except in 

 Newfoundland, but it is very common in the Baltic deposits of this 



