BRYOZOA. 261 
Mesotrypa discoidea. ] 
Zoarium discoid, base flattened, upper surface gently convex; diameter, so far 
as observed, less than 20 mm.; hight, sometimes divisible into two subequal layers, 
4 mm. or less. Upper surface with inconspicuous clusters of zocecia a little larger 
than the average. Zocecial apertures rounded, rather regularly arranged, about 
eleven in3 mm. Mesopores numerous, rather small, occupying merely the triangu- 
lar or quadrangular interspaces left between the adjoining rounded zocecial walls. 
The latter are very thin. As usual with species of this and related genera the 
mesopores are more abundant (in this case completely isolating the zocecia) in the 
basal or primitive,part of the zoarium than in the fully matured superficial portion. 
Acanthopores apparently absent. 
In vertical sections the chief peculiarity of the species is found in the tabulation 
of the tubes. In both sets of tubes, namely, diaphragms are present in unusually 
large numbers, there being a few more or less than thirty-five in 1 mm. in the meso- 
pores, while in the zocecial tubes the average is eight or nine in the same distance. 
In the latter the diaphragms, especially in the lower part, are frequently curved or 
oblique, as shown in the figure, but some of the tubes have practically horizontal 
diaphragms throughout. The mesopores are quite abundant at the curved basal 
part of the zowcial tubes, becoming, however, very much less so soon after these 
assume an erect position. Many of the zocecial walls may appear as completely in 
contact, but the rule is that at intervals, at any rate, they separate, producing the 
periodic tabulated swellings shown in the figure. 
This species, having no acanthopores, belongs near M. quebecensis Ami, sp., from 
which it differs strongly in vertical sections, the diaphragms being much fewer in 
that species. M. rotunda, also without acanthopores, has more numerous and com- 
paratively loosely tabulated mesopores. 
Formation and locality—Rare in the uppper part of the Galena shales (Anastrophia beds) at local- 
ities in Goodhue county, Minnesota. 
