66 AMERICAN FOSSIL BEYOZOA. [bull. 173. 



well represented — the Ctenostomata scarcely at all — but this may be 

 because ctenostomatous br3^ozoa rarely admit of preservation as fos- 

 sils. Of the Ctenostomata only six species have been described, all 

 Eopaleozoic; but about twenty other undescribed species are known, 

 some of which are from Neopaleozoic formations. Among the Cyclo- 

 stomata the Ceramoporid» and Diastoporidaj predominate during the 

 Trenton, the Monticuliporidaj and Trematoporid^e among the Trepo- 

 stomata, the Arthrostylidaj and Rhinidictyonida? among the Crypto- 

 stomata. 



The succeeding period, the Cincinnati, is very similar to its prede- 

 cessor in its bryozoan fauna, the various suborders maintaining about 

 the same relative representation. About one hundred undescribed 

 species are known, and it is probable that many species still await dis- 

 cover3\ The predominant families are the Ceramoporidte among the 

 Cyclostomata, the Monticuliporidte and Heterotrypid^ among the 

 Trepostomata, and the Ptilodictyonidaj and E-hinidict3"onida3 among 

 the Cryptostomata. 



The Anticosti group forms the connecting link between the Ordo- 

 vician and Silurian eras. In this formation the Trepostomata will be 

 found fairly numerous. A few species have been described by Bill- 

 ings, but were so meagerly characterized that we are unable to place 

 them generically, and hence have omitted them from our tables. The 

 Cryptostomata appear to predominate, though when the fauna shall 

 have been thoroughly studied the Trepostomata ma}^ be found to be 

 still slightly in the lead. 



From the Medina, which is practical^ limited to the State of New 

 York, no bryozoa have been made known. The Clinton contains but 

 few bryozoa, and these are generally not well preserved. The Trepo- 

 stomata have now dropped from first to second place, the Cryptostomata 

 are very much in the lead, the Cyclostomata rare. The Ptilodictyo- 

 nidaj now reach their maxinmm and are the predominant family both 

 in number of species and in individuals. 



With the Niagara a decided change has come. Large forms are com- 

 paratively rare; even among the Trepostomata the tendency to diminu- 

 tiveness is very noticeable. The Cryptostomata greatly predominate; 

 the Cyclostomata and Trepostomata are about equall}^ represented. 

 The Ceramoporidffi are giving place to the Fistuliporidaj, probably 

 their descendants. Among the Trepostomata the Batostomellidse, the 

 most enduring family of the br\' ozoa in Paleozoic time, predominate. 

 Among the Cryptostomata the Fenestellidee lead, and from this time 

 on are the chief family. - 



The Lower Helderberg is similar in its bryozoan fauna to the Niag- 

 ara, but the Fenestellidffi have been increasing rapidly and now form 

 nearly one-third of the bryozoa. No Cyclostomata except those belong- 

 ing to the Fistuliporidae and Ceramoporidse have been recorded. 



