BRYOZOA. 149 
Pachydictya. ] 
Section.a: Species in which the zoarium is not limited, and maculw or clusters of 
large zowcia are present. 
Pacuyprerya FoLIATA Ulrich. 
PLATE IX, FIGS. 1-5; PLATE X, FIGS. 5-10. 
Pachydictya foliata ULRICH, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Surv. Minn., p. 73. 
Zoarium growing from an attached basal expansion into erect, thin fronds, 
undulating and simple, or dividing palmately or irregularly ; both sides cellulife- 
rous; attaining a hight and width of 50 mm. or more, but specimens larger than 25 
mm, square are rare ; usual thickness about 1.2 mm., but in some old examples it is 
quite 3.0mm. Margin of fronds acute or rounded, often with a distinct non-periferous 
border. At intervals of 3.5 or 4.0 mm. the surface presents solid, substellate spots or 
“macul, that in most cases are on a level with the general plane of the surface, in 
others occupying the summits of low monticules, while in rare instances they may 
be even slightly depressed. These macule usnally appear smooth, but when well 
preserved are seen to be finely grano-striate. Zocecial apertures large, oval, arranged 
in regular diagonally intersecting series, in which fourteen or fifteen of the average 
size is the usual number in 5 mm. In the immediate vicinity of the macule they 
are larger, attaining a size of 0.4 mm. by 0.3mm., the average size in the spaces 
between the maculz being about 0.3 mm. by 0.2mm. There is a slight difference 
also in the size of the apertures of the old and young specimens, they being largest 
in the latter. Interspaces usually of less width than the zocecial apertures, concave 
and forming a distinct peristome around the aperture in the young examples ; becom- 
ing flattened and even faintly conyex, also minutely granulose with age. Interstitial 
vesicles seen at the surface in the youngest specimens only. 
In vertical sections the zocecial tubes arise rather abruptly from the mesial ]am- 
ine, the course to the surface throughout being also unusually direct. The prostrate 
or primitive cells may be in contact, with a thin divisional wall; but this is not the 
rule since the interstitial vesicules are developed at the same time. The character 
of the latter is clearly preserved for a distance of about 0.5 mm. on each side of the 
‘mesial lamin, but beyond this they are filled with solid material in which they are 
butiily traceable. Occasionally it is possible to detect faint dark lines passing vertically 
through this solid filling, indicating that communication was maintained with the 
horizontal median tubuli. The zocecial tubes are bordered on each side by a double 
line, and crossed, according to age, by from one to five complete diaphragms. These 
occur approximately on the same level in all the tubes, and at intervals correspond- 
ing more or less nearly with the diameter of the tube. If my view is correct, each 
