THE BRYOZOA BASSLEE. 375 



works on the structure of the Cheilostomata have been issued by 

 Harmer, and Waters since 1878 has been a most important, contributor 

 to this subject. His many memoirs on both Cenozoic and Recent bry- 

 ozoa likewise are of the highest value. In 1909 Levinsen 37 published a 

 memoir which is indispensable to the modern student. 



The fossil Cheilostomata also form the subject of numerous 

 researches, among which the work on Cretaceous faunas by D'Orbigny 

 and various monographs on the Tertiary of Europe/ 8 Africa, 39 and 

 South America 40 by Canu and of North America 41 by Canu and 

 Bassler should be mentioned. The last-named work contains numer- 

 ous text figures illustrating family and generic structure, in addition 

 to detailed references to the literature. 



DISTRIBUTION OF THE BRYOZOA. 



The continent of North America is undoubtedly the most favored 

 part of the earth for reading Paleozoic history, and it is equally 

 favored for the study of fossil bryozoa, as many of the Paleozoic 

 marine limestone and shale formations abound in these organisms. 

 The Eurasian land mass presents many surface exposures of Paleo- 

 zoic age, but they are to a greater or less extent disconnected, and the 

 fossil forms are not so well known as in America. In Asia the Salt 

 Range of India has yielded Carboniferous bryozoa, while in Europe 

 the region of the Ural Mountains and the areas bordering on the 

 Baltic Sea, England, and Scotland contain most of the Paleozoic 

 strata which have thus far afforded specimens. Three or four times 

 as many species have been made known from the Paleozoic of North 

 America as from all the rest of the world. 



The earliest undoubted bryozoan is a species of the peculiar trepo- 

 stomatous genus Nicholsonella, occurring in the Canadian rocks of 

 Arkansas, although a species referred doubtfully to the Ctenostomata 

 has been described from the Ungulite sandstone of Early Ordovician 

 age in Esthonia. The limestones and shales of the various divisions of 

 the Ordovician above the Canadian abound in stony brj^ozoa of the 

 Cryptostomata and TrejDostomata (pi. 1, fig. 1), although the Cyclo- 

 stomata are represented and an occasional species of Ctenostomata 

 may be found. In the Silurian, bryozoa are not so common, and the 



37 Levinsen, G. M. R. Morphological and systematic studies on the Cheilostomatous 

 Bryozoa (1909). 



33 Canu, F. Bryozoaires des terrains tertiaries des environs de Paris. Annales de 

 Paleontologie, tomes 2 (1907), 3 (1908), 4 (1909), and 5 (1910). 



39 Canu, F. Etude eomparee des Bryozoaires Helvetiens de l'Egypte avec les Bryozoaires 

 rivants de la Mediterran€e et de la mer Rouge. Memoires de l'lnstitut Egyptien, tome VI, 

 1912. 



40 Canu, F. Iconographie des Bryozoaires fossiles de l'Argentine. Anales del Museo 

 Xacional de Buenos Aires, tamo XVII (190S), tomo XXI (1911). 



41 Canu and Bassler. North American Early Tertiary Bryozoa. Bulletin 106, U. £5. Na 

 lional Museum, 1920. 



