46 BULLETIN 77, UNITED STATES NATIONAL MUSEUM. 



Geological Survey/' 1907, page 736. As detailed directions are given 

 in each of these volumes, it is unnecessary to repeat them here. 



It is of course needless to make tliin sections of every specimen. 

 After a species has been determined, the internal characters of other 

 specimens believed to belong to the same species can be determined 

 by slightly etching with acid whatever parts one wishes to study, 

 and, after moistening the etched areas, examining them under a 

 hand lens. With tliis very simple method, the structural details can 

 be seen distinctly. Comparison then with sectioned material or with 

 species which have been described and figured will in most cases 

 serve to establish the identity of the specimen. 



Most of the terms employed in the description of fossil bryozoans 

 are defined in the following notes on terminology : 



TERMINOLOGY. 



Zoarium. — The composite structure formed by repeated gemma- 

 tion. Although fairly constant for each species, the form wliich 

 results is very variable. Gemmation in a plane produces unilaminar 

 sheets, which are sometimes free, but are often parasitic (incrusting) 

 upon other organisms ; in the former case the protecting covering on 

 the under side is the epitheca. Hollow branches lined with an 

 epitheca are a special form of this mode of gemmation. Two 

 unilaminar sheets growing erect, back to back, form a hilaminar or 

 hifoliate expansion or frond. The epithecae of the two layers of 

 zocecia thus brought together form a mesotheca (mesial or medial 

 laminae). The small pores seen in the mesotheca, or between the 

 walls of adjoining zooecia of certain species when tliin sections of 

 well-preserved specimens are examined under the microscope, have 

 been given the name of median tuhuli by Ulrich. 



Zocecium. — The cavity inhabited by the animal. The bounding 

 wall is constructed of laminated tissue. In particularly well-pre- 

 served specimens, thin sections reveal very small tubular passages 

 penetrating the walls of adjoining zooecia. To these Ulrich has 

 applied the name communication pores. As will be noted in the 

 descriptions of certain species in this work, the Ceramoporidae show 

 especially well-marked pores of this nature. 



In some forms, especially among the Trepostomata, the zooecia 

 have the form of elongated tubes, which are crossed by partitions 

 termed diaphragms. The general term tahulse is sometimes em- 

 ployed in place of diaphragms. The opening upon the surface of the 

 zooecium or of the vestibule among the Cryptostomata is the aper- 

 ture. It is often closed by a zocecial cover (closure or operculum). 



One side of the zooecial cavity in some of the Trepostomata is lined 

 with a series of superimposed vesicles, the cystipJiragms; their purpose 

 or use is unknown. 



