THE BRYOZOA — BASSLER. 359 



ler of 1920. Hincks' British Marine Polyzoa (1880), Busk's Cata- 

 logue of Cyclostomatous Polyzoa in the British Museum (1875), 

 and his Challenger Expedition Report (1886) as well as numerous 

 papers by Waters, Smitt, Harmer, Canu, and other authors treat 

 of the recent Cyclostomata. 



The study of many Cyclostomata particularly those forming solid 

 calcareous zoaria requires thin sections. The preparation of such 

 sections is discussed in this article under methods of study. 



In addition to the ovicells and other features just mentioned, the 

 size of the orifices and the distances between them are important in 

 specific identifications. Probably the simplest and most trustworthy 

 method of identifying closely allied species is by the preparation 

 of uniformly magnified photographs of the zoarial surface. The 

 magnification of 12 and 25 diameters for the Cyclostomata has been 

 found most useful and is recommended for comparative purposes. 



ORDER 3. TREPOSTOMATA. 



This order is limited to the Paleozoic era when it flourished in a 

 wealth of species forming stony colonies which contributed largely to 

 the formation of many limestone strata. These colonies were always 

 calcareous and consist of masses, sometimes of considerable size, com- 

 posed of long coherent, prismatic, or cylindrical tubes with terminal 

 orifice. Each tube is composed of an inner or axial region with thin 

 walls and an outer, peripheral zone with thicker walls and compli- 

 cated structure. This change in the character of the tubes which 

 gives the name to the order (trepos, change) is accompanied by the 

 development of other features, namely, mesopores, acanthopores, 

 more numerous diaphragms and similar structures of the more ma- 

 ture zooid. 



The Trepostomata include the greater portion of the so-called 

 Monticuliporoids which for a long time were regarded as corals, 

 although their bryozoan nature was long insisted upon by Ulrich 

 who published many proofs of their affinities to undoubted bryozoa. 

 This relationship has been strengthened by the discovery by Cumings 

 that the budding plan of certain Ordovician genera is precisely the 

 same as in typical recent bryozoa, namely, that it consists of (1) a 

 protoecium or minute circular disk, (2) the ancestrula, a tubular 

 zooecium of the type seen in the Cyclostomata, and (3) several pri- 

 mary buds arising from and adjacent to the ancestrula. These primi- 

 tive structures are separated from the rest of the colony by a con- 

 siderable thickening of their posterior walls. In the corals, develop- 

 ment from the larva is direct the moment it becomes sedentary and 

 therefore the presence of the protoecium alone is practically con- 

 clusive as to the S3'stematic position of the Trepostomata with the 

 bryozoa. 



