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 zocecia 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, ete. 
2. The laminar zoarium, is a thin, free expansion, having the lower side covered 
by a wrinkled epitheca. Examples, many species of Fstulipora, ete. 
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 disc; 
or it may be conical. The under side is concave or flat and covered with an epi- 
thecal crust. Examples, species of Prasopora, Mesotrypa, and Leptotrypa discoidea 
Nicholson. 
5. The bdifoliate 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 Rhinidictyonidw and Ptilodictyonide. 
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. 
