NO. 3 OSBURN : EASTERN PACIFIC BRYOZOA CYCLOSTOMATA 615 



absorption of kenozoids around a fertile zoid. Heteroporidae, etc. 

 (Heteroporina Borg, 1944). 



Cerioporina Hagenow, 1851. 

 5. Zoarium adnate or short stipitate, discoid or semiglobular, sometimes 

 complex by the formation of subcolonies ; zoids radiating in all direc- 

 tions from the center and separated by alveoli (cancelli). Brood- 

 chamber zoarial by the fusion of alveoli around a fertile zoid. Lich- 

 enoporidae. ( Calyptrostega Borg, 1926). 



Rectangulata Waters, 1887. 

 In this order the older workers based their descriptions and classifi- 

 cation almost solely on zoarial characters, and even Hincks in the 1880s 

 paid little attention to the ovicells. Waters insisted on the importance 

 of the reproductive characters and Harmer, Calvet, Canu and Bassler, 

 Marcus, Borg, Silen and others, including the writer, have accepted 

 this point of view. 



The difficulty with zoarial characters is their variability, depending 

 partly on their adaptation to the substratum and other features of the 

 environment, and partly on the stage of growth. In the Crisiidae, the 

 younger stages are so much alike that, in the absence of ovicells, the 

 determination of the species is often impossible. 



In the encrusting species the nature of the substratum may determine 

 the size and form of the zoaria, and the environment often modifies the 

 appearance of erect species. Among the Tubuliporidae, encrusting species 

 are usually flat and regular on flat surfaces, but when the same species 

 develops on a small stem the zoarium may be variously contorted. In 

 deeper, quiet water, erect species are usually more slender and more 

 elongate, sometimes giving quite a different zoarial appearance from the 

 same species in the surf area along shore. In protected areas the peri- 

 stomes are usually much more elongate, and in crowded areas or on rough 

 surfaces the zoaria may be much reduced in size. Numerous "species" 

 have been described on such differences. 



By far the most constant characters in this order are found in the 

 ovicells or brood-chambers, either zoidal or zoarial. In the Hetero- 

 poridae and Lichenoporidae the brood-chambers are interzoidal or zoarial 

 spaces surrounding a gonozoid and their position in the zoarium and the 

 gonopores and their tubes (ooeciostomes) are fairly constant. In all the 

 others the ovicell is an expanded gonozoid, sometimes only slightly en- 

 larged, or again it may be greatly expanded over a considerable portion 

 of the zoarial surface and may surround some of the autozoid tubules. 

 There may be marked variation in the size and form of these ovicells, 



