2 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I39 



U. S. National Museum collections and are conspecific with Hall's pri- 

 mary types of the species. After seeing the sections, Ulrich greatly 

 restricted the concept of the genus and indicated the great range of 

 forms that Hall had included in the genus. The concept established 

 by Ulrich in 1883 has remained essentially unchanged to the present 

 time and was the type-genus concept for the family Trematoporidae 

 in 1889. Under Ulrich's definition of the genus, 12 species and sub- 

 species have been assigned to Trematopora, ranging in age from 

 Middle Ordovician through Middle Silurian. 



The primary type specimens were made available for sectioning and 

 study by N. D. Newell of the American Museum of Natural History. 

 Helpful suggestions were made by Helen Duncan and W. A. Oliver, 

 Jr., of the U. S. Geological Survey, and N. Spjeldnaes, of the Uni- 

 versity of Oslo. Thin sections were prepared by T. M. Robison of the 

 U. S. Geological Survey. Photography was done by J. Scott, and the 

 text figure was drawn by L. B. Isham, both of the Department of 

 Geology of the U. S. National Museum. 



INTERPRETATION OF SKELETAL MICROSTRUCTURE 



The skeletal structures of most trepostomatous Bryozoa are com- 

 posed of finely laminated calcite (fig. 1 and pi. 2). These laminae are 

 assumed to have been deposited parallel to the surface of the secreting 

 tissue (Cumings and Galloway, 1915, p. 361). Therefore, trends of 

 the laminae within skeletal structures such as walls and diaphragms 

 are considered to reveal something of the disposition of the original 

 secreting tissue and the mode of growth of the skeletal structures. 



In longitudinal thin sections of T. tuberculosa, laminae are com- 

 monly oriented parallel to the zooecial walls (fig. 1) in the endozone 

 (immature or axial region of authors) and to the thinner walls and 

 mesopore diaphragms in the inner region of the exozone (mature 

 region of authors). This type of microstructure is here designated 

 longitudinally laminated structure. Such an orientation of laminae 

 is assumed to indicate that the depositing tissue was parallel to the 

 walls and diaphragms, but it does not indicate whether the laminae 

 were deposited on one or both sides of the structures. 



Another type of structure is characterized by laminae that are 

 curved or angled transversely to the walls and diaphragms as seen in 

 longitudinal sections. The transverse laminae form V- or U-shaped 

 patterns with apices pointing distally and aligned along the median line 

 of a wall or diaphragm. This type of microstructure is here designated 

 transversely laminated structure. In T. tuberculosa, this structure is 



