126 THE PALEONTOLOGY OF MINNESOTA. 
[Rhinidictya. 
tudinally, about seventeen zocecia in5 mm. Except in a variety to be considered 
presently, the zoccial apertures always appear as direct. 
Vertical sections (plate VI, figs. 3 and 5) show that the zocecial tubes, in their 
course from the basal (median) plate to the superficial aperture, form an angle of 
about 50 degrees with the surface. In the primitive portion of the zoccia (7. e. that 
part which lies on each side close to the median lamin) the posterior side curves out- 
ward and forward so as to form a curve about equaling one-fourth of a circle. The 
anterior extremity of the curve terminates abruptly at the primitive aperture ; from 
this point to the surface of the zoarium, or in what has been described as the “ vesti- 
bular portion of the zocecium,” the course of the wall is nearly straight. In a few 
species of this genus (e. g. R. fidelis and R. minima) the junction between the “ves- 
tibule” and the curved posterior side of the “primitive cell”, is prolonged into a 
short septum that I have designated as the “superior hemiseptum.” In R. mutabilis, 
however, this septum is but little, if at all, developed, the junction being merely an- 
gular. When the section shows the transverse interspaces (this is often the case 
because of the great thickness of the interspaces in the vestibular region) they will 
be seen to exhibit interrupted dark vertical lines. These represent the tubular 
internal extensions of the minute granulations noticed at the surface. Stages in 
the growth of the zoarium may also be determinable. These are marked by dark 
lines, sharpest in the inner portions of the zoarium. 
In transverse sections the chief point of interest is the row of minute tubuli 
that exists between the two parts of the duplex mesial lamina. (See plate VI, fig. 6.) 
Tangential sections present a variety of appearances depending (1) upon the age 
of the fragments sectioned, and (2) the depth beneath the surface represented in the 
section. Using an old example the section may be made, with judicious manipula- 
tion, to show all the conditions through which the zoarium has passed, from the “ 
beginning of the zocecia on the mesial laminz to their mouths. Taking such a sec- 
tion, which, to be satisfactory, should not be less than 10 to 15 mm. long, the follow 
ing features are likely to result: Starting with the mesial lamin, which will be 
recognized as a faintly dark space, the first character worthy of notice are the “median 
tubuli.” These are represented by very delicate parallel lines, longitudinal in the 
central third of the zoarium, but gradually diverging or curving toward its edges in 
the lateral thirds. Though not yet clearly demonstrated in this species, I neverthe- 
less assume it to be a fact (because of observations in other forms possessing such 
tubuli) that the “mesial tubuli” connected with the minute tubes between the walls 
of the zocecia, the surface extensions of which have been described as granules. (See 
plate VI, fig. 18.) Just above the mesial lamine the section presents the basal or 
