502 E. O. ULRICH REVISION OF THE PALEOZOIC SYSTEMS 



Cambrian. Undoubted cystids are imperfectly known in Ozarkian and 

 Canadian rocks. These remains occur in both Atlantic and Pacific de- 

 posits. In the Ordovician but few true cystids are known in Gulf of 

 Mexico faunas, but they became very abundant in the Arctic waters of 

 this age. The crinoids, however, seem to have originated during the 

 early Ordovician in the southern middle Atlantic, where the dominant 

 types, as expressed in the invading Gulf faunas of this age are Dendro- 

 crinidae, Heterocrinidae, and Glyptocrinidae. These spread rapidly into 

 the northern middle Atlantic, which became an important center of 

 crinoid development and dispersal, and from here, via the Baltic, into the 

 Arctic basin. The dominant Ordovician crinoids in the northern Atlantic 

 are Dendrocrinidae, Carabocrinidae, Hybocrinidae, and Ehodocrinidae. 

 During the Silurian the Gothland crinoids, like the corals, spread freely 

 through the Arctic and then southward in America to northern Illinois. 

 They extended also southward into England, where a slightly different 

 development obtained, and thence into the Atlantic to the Gulf of 

 Mexico and the Mississippi embayment. All the succeeding Paleozoic 

 crinoid faunas, so far as known, originated in and spread from the 

 Atlantic basins. Regarding the Pacific realm it is an astonishing fact 

 that, excepting a few Indian and Australian species, no good Paleozoic 

 crinoid has been described from it.®^ In fact all echinoderms seem to 

 have been rare in this faunal realm during the Paleozoic. 



The bryozoa seem to have originated in the south Atlantic or Carib- 

 bean Sea during the Canadian period, a species of Nichplsonella being 

 exceedingly abundant in rocks of this age in northern Arkansas. Be- 

 ginning with the Ordovician they spread rapidly northward to the Baltic 

 and Arctic regions. In the middle Paleozoic ages the bryozoa in these 

 two, northern and southern, basins developed along somewhat different 

 lines. In deposits of Pacific waters the class is almost entirely unknown 

 until after the Devonian when its genera had become decidedly cos- 

 mopolitan. 



The brachiopods began early in all of the major oceanic basins. It is 

 only in the matter of genera and minor groups of species that notable 

 geographic limitations are observable. Thus Poramhonites and Trim- 

 erella seem to be confined to Arctic waters, Leptoholus, Hebertella, and 

 Rensselaeria to Atlantic, and Richtofenia to Pacific. In the Ordovician 

 and Silurian many genera of brachiopods were common to the Atlantic 

 and Arctic basins that have not been found in Pacific deposits. Toward 



63 The Waverlyan crinoids found in Montana and New Mexico doubtless invaded these 

 areas from the Gulf of Mexico. 



