152 THE PALEONTOLOGY OF MINNESOTA. Sahel, 
[Pachydietyi. 
over the whole surface. There seems to have been a cessation of developement in 
some places, causing the formation of irregular furrows, in which the old zocecial 
apertures are partly closed by a sheet of dense material. Thin sections failed to reveal 
anything unusual, hence, we may safely assume that these specimens present merely 
an abnormal condition of the species. 
Of internal peculiarities brought out by tangential sections the most striking 
are, (1) the unusual brevity of the end spaces. In many cases these are so short that 
the outer lines of the ring-like walls of succeeding zocecia are often nearly in con- 
tact. Generally the length of these spaces is less or about equals half the transverse 
diameter of the zovecia ; (2).the continuous longitudinal lines of median pores (there 
is as a rule only one in each interspace between the rows of zocecia) appears more 
flexuous than usual; and (3) the macule or solid spots, which do not interrupt the 
course of the lines of median tubuli. A number of isolated tubuli, otherwise seem- 
ingly of the same nature, occur between the lines mentioned. 
In vertical sections the zocecial tubes frequently have diaphragms, their course 
to the surface is less direct than common, and the interspaces or walls unusually 
thin. , 
The growth and maculose surface distinguishes this species from the other Min- 
nesota forms of the genus, none of which are found, however, in the same beds with 
P. occidentalis. Though perhaps still to be regarded as intermediate in some respects 
between P. acuta Hall, sp.,and P. fenestelliformis Nicholson, sp., further investigation 
proves the relationship to those species to be more remote than I thought at first. 
It seems also to have preceded both in time. Compared with the first it is found to 
differ in its mode of growth, the zoarium being wider, in the character of the inter- 
spaces, and in the macule which are wanting in that species. The second has larger 
zocecia, and both present well marked internal differences. 
Formation and locality—Rather abundant in the upper third of the Trenton shales at St. Paul, 
Minnesota. A few specimens also from the same horizon in Goodhue county. 
Mus. Reg. Nos. 5949, 7646. 
Section b: Species in which the width of the zoarium is limited, and the margins 
subparallel. 
PacuypicTya FIMBRIATA Ulrich. 
PLAT® VIII, FIGS. 28-84; PLATE IX, FIGS. 13 and 14. 
Pachydictya fimbriata ULRICH, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Sur. Minn., p. 75. 
Zoarium rather small, ramose, the branches with subparallel margins, from 2 to 
5 mm. wide, averaging a little over 3 mm., thin, the thickness rarely exceeding 0.5 
mm.; bifurcations dichotomous, occuring at variable though generally at long inter- 
