EAELY PALEOZOIC BRYOZOA OF THE BALTIC PROVINCES. 75 



cated appearance to the zooecia. The mtemal structure in the 

 family is equally characteristic, in fact, so much so that the term 

 ''ceramoporoid structure" has its own definite meaning. Thm sec- 

 tions, magnified 20 or more times, show that the cell walls are 

 minutely porous and are composed of irregularly laminated and 

 intimately connected tissue. Microscopic examination is, however, 

 not necessary for a preliminary determination of a ceramoporoid 

 because the intimately connected tissue forming the cell walls, from 

 its very nature, will show no distinct fracture. If a trepostomatous 

 bryozoan be broken, the fracture will be clean-cut and clear, sepa- 

 rating adjoining cells, but in the case of a ceramoporoid, the break 

 will proceed through or across the walls indiscriminately, and present 

 a dull, structureless surface such as would be produced in an amor- 

 phous body. This style of fracture, however, is not limited to the 

 Ceramoporidee but is present in the FistuliporidjB and all the families 

 referred to the Cyclostomata. Undoubtedly this order of Bryozoa 

 must have had a different chemical composition from other orders 

 like the Trepostomata. The latter seem to contain more crystalline 

 calcite in their structure, while the Cyclostomata apparentl}^ have 

 m-ore corneous material as a part of their skeleton. Certain genera, 

 Ceramopora especially, distinctly show large pores piercing the walls 

 and thus connecting adjoining zooecia and mesopores. Good ex- 

 amples of such connecting pores are illustrated on the following pages. 

 The zooecia in the different members of the family are sometimes in 

 contact but more frequently they are separated by more or less 

 irregularly shaped mesopores which are free from any comiecting or 

 strengthening tissue. The Fistuliporidse, on the other hand, may be 

 said to differ principally in having the inter-zooecial spaces occupied 

 by vesicular tissue. Such vesicular tissue, although poorly devel- 

 oped, is sho^Mi in the primitive fistuliporoids described on later pages 

 as Fistulipora primseva and Chilotrypa immatura. 



Various methods of growth obtain in the Ceramoporidse, and in 

 some cases the shape of the zoarium constitutes a good generic char- 

 acter. Another genus, on the contrary, may include a variety of 

 forms of growth, in which case it must be distinguished by other 

 features such as the development and shape of the lunarium, the 

 occurrence of minute tubules traversing the lunarium lengthwise, 

 and the presence or absence of mesopores. These distinctive features 

 are cited under the discussion of the individual genera. The chief 

 characters of the family have been described by Nickles and Bassler 

 as follows: 



Zoarium variable; maciilse or clusters of mesopores or of zooecia larger than usual 

 at regular intervals; zooecia tubular, at first prostrate, continue obliquely or directly 

 to the surface, often with a few diaphragms; apertures commonly oblique, provided 

 with a lunarium; mesopores generally present, always irregular and usually without 

 diaphragms; walls minutely porous, formed of intimately connected and irregularly 

 laminated tissue. 



92602°— Bull. 77—11 7 



