102 THE PALEONTOLOGY OF MINNESOTA. 



[Introduction. 



The necessity of two sections, a vertical and a transverse, is at once obvious, but 

 as neither of these sections will give us a cross section of the zocecia in their periph- 

 eral region, where the adult and consequently the most important characters are to 

 be found, it is evident that a third section must be prepared, which will enable us to 

 investigate these characters. This section, which is called "tangential," must divide 

 the zoarium along a plane parallel with the surface, and only a little below it. 



Of bifoliate forms two tangential sections ought to be made, one passing through 

 the zoarium just below the surface, and the other just above the median lamina. In 

 thin examples of this style of growth one large section can be made to show the 

 characters of the zooecia from their origin to the aperture. 



For massive, parasitic, or discoid zoaria, two sections (vertical and transverse) 

 will ordinarily suffice to bring out the principal characters, but it is advised that 

 two or more transverse sections be prepared, dividing the zoarium at different hights. 



In beginning the study of Bryozoa the first essential is to learn to group them 

 according to their outer form and mode of growth. The outward form, though 

 extremely variable when the whole class is taken into consideration, is tolerably 

 constant for each species, and not infrequently all the species of a genus will adhere 

 more or less strictly to some particular method of growth. On the other hand many 

 very distinct types may assume very nearly the same outward form. But the dis- 

 crimination between these is a second step in the investigation. 



The zoaria will usually exhibit one or the other of the following conditions : 



1. The parasitic or incrusting zoarium, in which the colony is spread over foreign 

 bodies. Examples,' Ceramoporella, Spatiopora, Stomatopora, Berenicea, etc. 



2. The laminar zoarium, is a thin, free expansion, having the lower side covered 

 by a wrinkled epitheca. Examples, many species of Fistulipora, etc. 



3. The massive zoarium, may be of irregular or rounded form, free, attached at 

 the base, or grow around some foreign body. Examples, species of Monotrypa and 

 Monticulipora. 



4. The discoid zoarium has the form of a plano-convex, or concavo-convex distt; 

 or it may be conical. The under side is concave c^Hat and covered with an epi- 

 thecal crust. Examples, species of Prasopora, Mesotrypa, and Leptotrypa discoidea 

 Nicholson. 



5. The bifoliate zoarium, in which the zocecia diverge from a double median 

 lamina or basal plate, and open upon the two surfaces of a foliaceous expansion, or 

 of flattened branches. Examples, all the Ehinididyonidie and Pt'dodidyonid<i>. 



6. The dendroid or ramose zoarium, in which the entire free surface is celluli- 

 ferous, is very common among paleozoic Bryozoa. Examples, Batostomella, Bythopora. 

 Hemiphragma, Nematopora. 



