104 THE PALEONTOLOGYl OF MINNESOTA. 



[Introduction. 



character of the " acanthopores," "median tubuli," " lunarium," zooecial covers, and 

 " hemisepta," the disposition and character of the " diaphragms " and " cystiphragms," 

 and the minute structure and independence or amalgamation of adjoining zooecial 

 walls. 



Acanthopores may be small (plate XV, figs. 15 and 17) or large (plate XXIII, fig. 

 35), ^nd 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 laminae 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. Zooecial 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 chiefiy among the Cryptostomata. On plate VI, figs, la 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 a rule 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 zooecial 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 zooecial 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 Ceramoporidce and Fistuliporidce, 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 zooecial tubes, and the strength and character of the curve in which they 

 approach the surface. 



CLASSIFICATION. 



The class Bryozoa is divided by Kay Lankaster into two very unequal subclasses, 

 the Holobranchia, in which the lophophore, or ring of tentacles, is unbroken and con- 

 tinuous, and the Pterohranchia, in which it is divided into two plumed arms or pro- 

 cesses, bearing a resemblance to the branchial appendages of the Brachiopoda. 



