376 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1920. 



Cryptostomata developed at the expense of the Trepostomata. In 

 the Devonian and Carboniferous the Trepostomata became much 

 reduced in numbers and finally disappeared, while the Cryptostomata 

 formed a wealth of species, especially of the lacelike Fenestella (pi. 1, 

 fig. 2) and its allies. The Ctenostomata remain as sparsely repre- 

 sented as before, but the Cyclostomata have increased in number by 

 the development of the great family Fistuliporidae. 



With the beginning of the Aiesozoic a decided change occurs in 

 the bryozoa. The Cryptostomata and Trepostomata have disap- 

 peared entirely, the Ctenostomata are as rare as before, but the 

 Cyclostomata now develop great numbers of species, with zoaria quite 

 similar in many instances to the Paleozoic Trepostomata. The 

 Cyclostomata remain the predominating type until Upper Creta- 

 ceous time, when the Cheilostomata, which appear in the Jurassic, 

 now expand into so many species that they soon attain supremacy. 

 D'Orbigny alone has described not less than 537 species of Upper 

 Cretaceous Cyclostomata and 300 Cheilostomata, although many of 

 these are synonyms. This great development of the bryozoa in the 

 Mesozoic is known only in European strata, for in North America 

 and in other parts of the world these rocks have yielded compara- 

 tively few bryozoan faunas. 



Both North America and Europe are noted for their Cenozoic 

 bryozoan faunas. The Atlantic and Gulf coastal plains of North 

 America and the northern and southern slopes of the Alps, as well as 

 numerous other localities in Europe, are rich in bryozoa with the 

 Cheilostomata and Cyclostomata well represented and the former 

 predominating. Southern Australia likewise affords an abundant 

 Tertiary bryozoan fauna. 



In the recent seas the Cheilostomata, exhibiting the bryozoa at the 

 highest stage of their perfection and beauty, is the predominating 

 order and numerous species have been described from all the oceans 

 where they occur, usually in abundance, from tide level down to 

 great depths. The voyage of the Challenger brought forth a wealth 

 of species which has since been greatly augmented by various ex- 

 peditions, as well as by the activity of local collectors. The seaweed 

 tossed up so abundantly along certain coasts is a fertile collecting 

 place for many parasitic species of Cheilostomata and Cyclostomata. 



STRATIGRAPHIC VALUE. 



The use of fossil bryozoa in stratigraphic work has scarcely at- 

 tained the importance it deserves. In American Paleozoic strata 

 they are preeminently the fossils to be relied upon in correlation 

 work. They are nearly always abundant, and even when poorly 

 preserved exteriorly can be identified by thin sections. Crinoids and 



