184 THE PALEONTOLOGY OF MINNESOTA. 
[Stictoporella. 
equalling the diameter of the zocecial apertures. Arrangement variable, rather 
irregular, generally in diagonally intersecting series, with from fifteen to eighteen, 
but averaging sixteen in 5 mm, 
Internal structure, as shown in thin sections, very similar to that of S. angularis, 
the only differences noticed resulting from the much greater development. of 
mesopores. 
The much greater abundance of mesopores distinguishes this species from S. 
angularis, var. intermedia, while the frondescent habit of its zoarium separates it 
from all the other Lower Silurian species of the genus. Associated in the same 
strata are Pachydictya foliata and Phyllodictya frondosa ?, two bifoliate species likewise 
characterized by foliaceous zoaria. In other respects, however, they differ so obvi- 
ously from S. frondifera, that detailed comparisons are rendered unnecessary. 
Formation and locality.—Rather abundant in the lower beds of the Trenton shales, at Minneapolis, 
St. Paul, Preston, near Fountain, and other localities in the state where this horizon is exposed. 
Mus. Reg. Nos. 5945, 5947, 7650. 
STICTOPORELLA ORIBROSA Ulrich, 
PLATE X, FIGS. 21-25; PLATE XI, FIGS. 22 and 23. 
Stictoporella? cribrosa ULRICH, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Sur. Minn., p. 69. 
Zoarium growing from a small expanded basal attachment into thin erect 
fronds, perhaps 50 mm. in diameter, composed of branches that inosculate at short 
intervals till there is produced a broad expansion perforated at more or less regular 
intervals by circular or elliptical fenestrules. Both sides of frond celluliferous, 
consisting of two equal layers of cells grown together back to back in the manner 
usual with bifoliate Bryozoa. Fenestrules elliptical, sometimes circular, varying 
greatly, the average size about 1.38 mm. by 1.8 mm., or less than the width of the 
branches ; but the same frond may show extremes like 1 mm. and less, and 3.5 mm. 
Their arrangement is therefore more or less irregular, and taking other cribrose 
species of the genus into consideration, this irregularity in the size and distribution 
of the fenestrules is to be considered as a characteristic of the present species. 
Width of branches generally 2.0 mm. or 2.5mm. but varying between the extremes 
of 1.5 mm. and 3.1 mm. Zowcial apertures small, subcircular or elliptical, the 
average size about 0.1 mm. by 0.12 mm., set into sharply defined, polygonal or 
rounded, sloping areas, about nine in 2.5mm. Mesopores small, numerous, often 
completely isolating the zocecia. Generally, however, the latter are in contact with 
each other at limited points. Around the fenestrules there is a band, 0.5 mm. or 
less wide, occupied solely by mesopores. As in other species of the genus the 
