NICKLES AMJ UASBLKU.J GEOLOCjIlC Dl^STlUliUTIUN. 65 



in the Great Plains and Rocky Mountain rej^ions, and the territory 

 hordcrintr the Pacific. Comparatively few })ryozoa of later a^e hiive 

 been made known, thoug-h the indication.s are that a large number of 

 .species will be the reward of thorough collecting. In central and 

 southern Europe Mesozoic and Tertiary beds often abound in bryozoa. 



GEOLOGIC DISTRIBUTION. 



Though the materials for the liistory of the biologic and ph3'lo- 

 genetic development of the Paleozoic bryozoa are to be found in the 

 rocks of the North American continent, scarcel}^ a beginning has yet 

 ])een made. Before this history can ])e written with any degree of 

 correctness, the preliminary work of collecting, studying, classifying, 

 and comparing the faunas of successive formations must be well under 

 way. This work is still in its infancy. The only worker in this field 

 who has yet cared to do any generalizing is Mr. E. O. Ulrich, and he 

 has not ventured very far. 



A study of the tables given farther on will afford some indications 

 of the rich results which the future may be expected to bring. It is 

 not possible to use the numbers there given determi natively for gen- 

 eralizations, for while the tables give the numbers of all species and 

 genera thus far described and recognized as valid, there remains in 

 various collections, public and private, a very large number of unde- 

 scribed species and some new genera. Therefore, any conclusions 

 based upon the numbers in the tables can at best be merely tentative. 



No l)ryozoa have j^et been knoAvn from the Cambrian, or the lowest 

 of the Ordovician formations — the Calciferous. Whether this means 

 that thej'^ are actually absent, or that they are so rare as not yet to 

 have fallen under collectors' eyes, can not be said. From the Chazy 

 rocks come the oldest unequivocal bryozoa, three species of Phyllopo- 

 rina, one of Rhinidictya, and one of Stictopora, all cryptostomatous 

 genera, with indications of the presence of some Trepostomata. It is 

 probable that future collecting will very much increase this meager 

 list, though good localities seem to be rare. 



In the Trenton period the bryozoa occur in the greatest profusion. 

 The few Chazy forms have multiplied, as if by magic, into the large 

 Trenton fauna. One of the missing links in paleontological historj'^ 

 doubtless occurs at this point. The Trenton formation stands second 

 in the number of described species, though it will probably in the end 

 lead. Not less than 80 species are known that await description. The 

 Trepostomata form half the fauna. The Cryptostomata rank next. 

 The latter soon became and continued the predominant t^^pe of the 

 Paleozoic eon, 3'ielding their supremacy in later times to the Chilo- 

 stomata, which are represented douljtfuUy in the Paleozoic seas b\' the 

 single genus Paleschara. In the Trenton the Cyclostomata are fairly 



Bull. 1T3 5 



