104 THE PALEONTOLOGYjOF MINNESOTA. 
(Introduction. 
character of the “acanthopores,” “median tubuli,” “lunarium,” zoccial covers, and 
“hemisepta,” the disposition and character of the “ diaphragms” and “cystiphragms,” 
and the minute structure and independence or amalgamation of adjoining zoccial 
walls. 
Acanthopores may be small (plate XV, figs. 15 and 17) or large (plate XXIII, fig. 
35), and will generally have a very small cavity, but it may be comparatively large 
as in Batostoma (plate XXVII, fig. 10). 
Median tubuli may be present between the mesial lamine of bifoliate forms and 
between the erect portions of the zocecia (plate IX, figs. 5 and 12). 
The lunarium is shown in several types on plate 28. Zocecial covers usually 
have a small subcentral perforation; they may be smooth (plate XXII, fig. 23) or 
with a radial ornamentation (plate XXIII, fig. 26). The opening may be laterally 
situated, and is often closed. 
Hemisepta occur chiefly among the Cryptostomata. On plate VI, figs. 7a and 8 
represent good examples of the superior one, while fig. 20 on plate XIV, shows the 
appearance of the inferior hemisepta. 
Diaphragms may be remote or crowded (plate XXII, figs. 9, and 38), present in 
the axial region (see vertical sections on plate XXIII) or absent (plate XXVI, figs. 1 and 
29). As arule they are complete and straight, but they may be incomplete as in 
Hemiphragma (plate XXIV); and they are always more abundant in the mesopores 
than in the true zoccial tubes. Cystiphragms when present, occur in conjunction 
with the diaphragms. Usually they overlap each other, as in most of the vertical 
sections on plate XVI. Occasionally they are separated and appear as semicircular 
lines lining one or both walls of the zocecial tubes in vertical sections, as in fig. 16 
of the same plate. In other cases they are oblique or funnel-shaped, as in figs. 3, 4, 
and 5, on plate XVII, 
In most Bryozoa the walls of contiguous zocecia are strictly independent and sep- 
arable from each other, but in the Ceramoporide and Fistuliporide, they are com- 
pletely fused together. 
Among the remaining characters that are brought out by thin sections, it is 
important to observe the relative length and shape of the primitive or axial portion 
of the zocecial tubes, and the strength and character of the curve in which they 
approach the surface. 
CLASSIFICATION. 
The class Bryozoa is divided by Ray Lankaster into two very unequal subclasses, 
the Holobranchia, in which the lophophore, or ring of tentacles, is unbroken and con- 
tinuous, and the Pterobranchia, in which it is divided into two plumed arms or pro- 
cesses, bearing a resemblance to the branchial appendages of the Brachiopoda. 
