BRYOZOA. 261 



Mesotrypa dlscolrtea.] 



Zoarium discoid, base flattened, upper surface gently convex ; diameter, so far 

 as observed, less than 20 ram.; bight, sometimes divisible into two subequal layers, 

 4 mm. or less. Uppe'r surface with inconspicuous clusters of zocecia a little larger 

 than the average. Zooecial apertures rounded, rather regularly arranged, about 

 eleven in 3 mm. Mesopores numerous, rather small, occupying merely, the triangu- 

 lar or quadrangular interspaces left between the adjoining rounded zooecial 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 zooecial 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 zooecial 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. 



