232 THE PALEONTOLOGY OF MINNESOTA. 
(Homotrypella mundula. 
Internal characters: Vertical sections show that the peripheral region is narrow, 
that here, and in the turn of the zocecial tubes, the diaphragms are closely arranged, 
with from six to eight in0.5mm. Justin the turn of the tubes, along the upper wall, 
one, two or three cystiphragms were developed. The mesopores begin at the same 
point, and in these the diaphragms are more crowded and thicker. In the central 
part of the axial region the tubes are larger than farther out, and intersected by 
diaphragms at intervals of about 0.5 mm. In nearing the surface the tubes become 
narrower and the diaphragms closer. Tangential sections are distinctive in showing 
the wide interpaces, large and sharply-deftned mesopores, and the ring-like defi- 
nition of the zocecial walls. In most cases the zocecia are completely separated or 
rarely touch each other without sacrificing any of their roundness of outline. 
Compared with other species, the Homotrypa ? intercalaris of this work seems 
to be the nearest and distinguished chiefly in vertical sections, diaphragms being 
absent in the axial region of that species and cystiphragms more numerous in the 
peripheral. For further remarks on this relationship, see p. 216. For comparisons 
with H. mundula, see under that description. 
Formation and locality—Galena shales, near Cannon Falls, Minnesota. Two fragments from the 
middle Trenton shales at Minneapolis seem te be identical in nearly every respect, but so far the species 
has not been found in the upper third of the shales intervening between these two horizons. 
Mus. Reg. No. 8124. 
HomoTrRYPELLA MUNDULA, fh. Sp. 
Fig. 12. Homotrypella mundula Ulrich, Galena shales, Decorah, Iowa. 
a, tangential section; b and ¢, opposite sides of a vertical section, showing differences in the number 
of cystiphragms; all x 18. 
Zoarium dendroid, branches subeylindrical, small, 2.5 to 5.0 mm, in diameter, 
dividing, generally dichotomously, at irregular intervals. Monticules wanting, but 
in most cases clusters of zocecia, slightly larger and more widely separated than the 
average, are to be distinguished. Zocecial apertures somewhat irregularly distributed, 
