BRYOZOA. 159 
Pachydictya.] 
The small, dwarfish appearance of the zoarium of this species will distinguish it 
from all others of the genus known to me. In other respects the species resembles 
P. acuta Hall, which occurs associated with it, but because of the much smaller size 
of P. pumila confusion between them is rendered highly improbable. Another asso- 
ciated species, Rhinidictya minima, is more likely to be confused with it, but after a 
little comparative study, the student will find himself able to distinguish them 
almost at a glance. The rather rigid and subcylindrical character of the stems of 
the next described P. triserialis are sufficiently distinctive of that species, and render 
further comparisons unnecessary. 
Formation and locality.—Base of the Galena shales, near Cannon Falls, Minnesota, where it is asso- 
ciated with species of Nematopora, Arthroclema armatum, Helopora mucronata, and other small Bryozoa, 
all of them characteristic of the horizon. A single example apparently referabie to this species, was found 
at the horizon of Phylloporina corticosa, and another occurs on a slab of Trenton limestone, from Trenton 
Falls, New York. 
Mus. Reg. No. 8107. 
PACHYDICTYA TRISERIALIS Ulrich. 
PLATE X, FIGS. 1-14. 
Pachydictya triserialis ULRIcH, 1890. Jour. Cin. Soc, Nat. Hist., vol. xii, p. 187, fig. 12. 
Zoarium consisting of very slender, parallel-margined, subeylindrical or com- 
pressed branches, 0.6 to 0.8 mm. wide, and 0.3 to 0.55 mm. thick ; branches bifurcat- 
ing at intervals of from 5 to 10 or more mm., oval or obtusely hexagonal in cross- 
section, the margins never, or at any rate but rarely, acute, in most cases to be 
described as narrowly rounded. Each face with three rows of longitudinally 
arranged zocecial apertures, occasionally with a fourth row, but only for a short 
distance. These rows are often not exactly parallel with the margins of the 
branches, a faint tendency to arrangement in long spirals being perceptible in 
those cases. Zocecial apertures elliptical, nearly twice as long as wide, largest in 
young or worn examples, separated by intervals equal to their long diameter, with 
from 11 to 13 in 5 mm.; occasionally enclosed by a delicate rim or peristome, but 
oftener with sloping edges. Between the rows an obtuse ridge. LHntire surface, 
especially of the older portions, minutely papillose. Non-poriferous margins nar- 
row, readily overlooked, generally wider on one side than on the other. 
Internal characters similar to those of P. acuta Hall, and P. pumila. In such a 
small species the interstitial vesicles are necessarily reduced to a minimum, and 
in this one the solid filling of the interspaces is so dense that their original presence 
is not easy of demonstration. 
The subhexagonal narrow branches of this species present considerable resem- 
blance to species of Nematopora like N. lineata (Helopora Billings). Of course, there 
