160 THE PALEONTOLOGY OF MINNESOTA. 
[Trigonodietya. 
is no real affinity between them, this being, as is clearly shown by transverse sec- 
tions, a bifoliate zoarium, while in Nematopora the zocecia diverge equally in all 
directions from the center of the branch. Iam not acquainted with any species of 
Pachydictya, nor with any associated species of bryozoan, with which the slender 
ramulets of P. triserialis might be confounded. 
Formation and locality.—As yet known only from the Trenton limestone at Montreal, Canada, but 
it is not at all unlikely that the species is to be found in the Minnesota equivalent of that horizon, 
Genus TRIGONODICTYA, n. gen. 
Zoaria with triangular branches, constructed upon the plan of Prismopora, but 
with zocecia and all minute details of structure precisely as in Pachydictya. 
Type: Pachydictya conciliatrix Ulrich. 
Another species occurs in the Clinton rocks near Eaton, Ohio, which, because it 
is the only bryozoan with triangular branches known to me from Upper Silurian 
strata, and may therefore be distinguished from associated forms with ease, I pro- 
pose to name T'rigonodictya eatonensis, n.sp. It is rather more slender than the Trep- 
ton species, and its branches divide at less frequent intervals. The three surfaces 
are also flat instead of concave, while in thin sections the interspaces. between the 
comparatively large oval Zocecia are thinner, and the lines of erect median tubuli 
much less distinct and not so numerous. 
TRIGONODICTYA CONCILIATRIX Ulyich. 
PLATE IX, FIGS. 11 and 12; PLATE X, FIGS. 15-20. 
Pachydictya conciliatric ULRIcH, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Surv. Minn., p. 76. 
Zoarium of irregular growth, dividing at frequent intervals, consisting of equal- 
sided triangular branches, with the three faces concave, each averaging about 3 mm. 
wide; or of more or less rapidly spreading, small, flabellate fronds, with from one to 
five salient, divaricating ridges on only one or both sides. All intermediate condi- 
tions between these two extremes occur. Each of the surface ridges has a non- 
poriferous, sharp summit, and, beginning as a mere line, it rises gradually until it is 
sufficiently high to permit of the formation of a new triangular branch, when it 
forms one of its edges. Zoccial apertures elliptical, slightly oblique, smallest and 
arranged longitudinally over the central half of each face; here with 12 or 13 in 5 
mm., a faintly elevated line between the rows, and the width of the longitudinal and 
lateral interspaces generally about equal to the respective diameters of the aper- 
tures. ‘Toward the non-poriferous edges the apertures are directed obliquely upward 
and outward, and increase in size gradually till those in the outermost row are quite 
