146 THE PALEONTOLOGY OF MINNESOTA. 
[Pachydictya. 
sented by the marginal rows of apertures which are commonly of larger size, with 
wider interspaces, and less regularly arranged than those of the central rows. Zocec- 
ial tubes rising rather abruptly from the mesial lamine, the primitive cells with thin 
walls, longitudinally arranged, of elliptical, semicordate, or subquadrate form, in 
most cases partially separated from neighboring cells by small interstitial vesicles. 
Toward the surface their walls are thickened, often ring-like, subelliptical in cross- 
section, usually completely isolated, the interspaces solid excepting that they are 
transversed by one or more, straight or flexuous, series of minute tubuli. One or 
more (the number depends upon age of example) complete diaphragms in each 
zocecial tube. Apertures usually elliptical, rarely subangular, the “closures” with a 
subcentral small opening. Interspaces grano-striate, concave and forming a peri- 
stome about the zocecial apertures, or thrown up into longitudinal ridges. Median 
tubuli between the halves of the double mesial plate. 
Type: P. robusta Ulrich. 
The distinguishing characters of section a, which includes the type of the genus, 
are (1) the wide, palmate or foliar zoarium, and (2) the macule and clusters of large 
zocecia. The section might be still further subdivided according to whether the 
longitudinal arrangement of the zoccial apertures predominates, or that in diagon- 
ally intersecting series. The latter would include the species robusta, everetti, foliata, 
magnipora and hexagonalis, all, save the last, described by me from the lower beds of 
the Trenton formation; while the former would embrace the species occidentalis 
Ulrich (upper Trenton), fenestelliformis (Nicholson), firma, gigantea, and splendens, 
Ulrich (upper beds of Hudson River group), and species obesa and turgida, described 
by Foerste from the Clinton rocks of Ohio. 
In section b, the zoarium is narrow, and its margins subparallel, while the longi- 
tudinal arrangement of the zocecia is always the predominating one. It seems that 
macule, or merely an unusual width of the interspaces, must always accompany the 
clusters of large cells, and as the room was insufficient in these narrow zoaria for 
their proper development, or, it may have been that their presence would have 
interfered too greatly with the regular growth of the branches, they (the large cells) 
are instead arranged along the margins, where we may assume, the necessary condi- 
tions to have been afforded by the non-poriferous border, which is constructed essen- 
tially upon the same principle as the maculze.* 
The following species are to be arranged under Section b: acuta (Hall) fimbriata, 
pumila, and triserialis, from the Trenton ; alcyone, arguta, and rustica, of Billings, from 
*It is a fact worth remembering that as soon as the width of the zoarium of one of the paleozoic bifoliate Bryozoa 
exceeds 4 or 5 mm., a maculum or cluster of cells larger than the average is found a short distance beneath the axes of 
bifurcation. A still greater increase and we have a row of macule or monticules along the center of the surface, Several 
instances of this kind are illustrated on the plates accompanying this volume. (See plates VII and VIII.) 
