BRYOZOA. 135 
Rhinidictya.] s 
superior hemiseptum will distinguish the sections at once from those of all other 
species except P. minima. That species occurs at a higher horizon (Galena shales), 
grew differently, has smaller elliptical zocecial apertures and much thicker, as well 
as quite differently marked interspaces. 
Formation and locality.—Rare in the lower third of the Trenton shales at Minneapolis, Minnesota. 
Mus. Reg. Nos. 5936, 5937. 
RHINIDICTYA TRENTONENSIS Ulyich. 
PLATE VI, FIGS. 14-18; PLATE VII, FIGS. 6-9. 
Dicranopora trentonensis ULRICH, 1882. ‘‘ Amer. Pal. Bry.,” Jour. Cin. Soc, Nat. Hist., vol. v, 
p. 160, pl. 6, figs. 15, 15a. 
Stictopora fidelis (part.) ULRICH, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Sur. Minn., p. 68. 
_ Zoarium branching dichotomously at intervals of from 8 to20 mm. Branches 2 
mm., or a little less, wide, sharp-edged, the non-poriferous margin very narrow 
Zocecia in from eight to eleven ranges, nine or ten the commonest numbers. Aper- 
tures nearly direct, comparatively large, of elliptical, subquadrate or hexagonal form, 
with sixteen, rarel yseventeen, in 5 mm. longitudinally, and five in 1 mm. transversely; 
those forming the marginal row usually a little larger than the average and directed 
slightly outward. Interspaces thin, apparently without granules, the longitudinal 
ones but little, if at all, elevated over those running transversely, the former gene- 
rally a little zigzag in their course. - 
In tangential sections dividing the zoarium just beneath the surface, the inter- 
spaces are moderately thick, and contain a line of very minute pores running length- 
wise between the rows of cells. Here the zocecia, or rather their “vestibular” por- 
tions, are elongate-elliptical, but at a deeper level, where the section cuts down into 
the primitive portion of the zoarium, they have the usual oblong-quadrate, or sub- 
rhomboidal shape. In one of the sections showing this region (see pl. VI, fig. 18) 
a row of “median tubuli” is distinctly visible in the transverse partitions. 
Vertical sections remind us much of Ff. nicholsoni and R. grandis, in this, that the 
interspaces or walls are rather thin, and that there is not even a sign of a superior 
hemiseptum at the base of the “vestibule,” the walls being merely thickened a little 
abruptly. In sections of thick examples a complete diaphragm may cross the tubes. 
In such cases it is common to find each half of the zoarium, in part at least, to con- 
sist of two superimposed layers of cells. 
A re-examination of the Tennessee type of this species has shown conclusively 
that it is not a Dicranopora but a Rhinidictya, with relations to PR. nicholsoni and R. 
grandis, its systematic position being nearly intermediate between them. From the 
