140 PALEOZOIC PALEONTOLOGY. 
MOLLUSCOIDEA. 
BRYOZOA. 
PRASOPORA SIMULATRIX Ulrich. 
Plate VIII., Figs. 1-38. 
1886. Prasopora simulatrix Ulr., 14th Ann. Rep. Geol. Nat. Hist. 
Surv. Minn., p. 85. 
1895. Prasopora simulatrix Ulr., Pal. Minn., pt. I., p. 245, pl. 16, 
figs. 1-10. 
“Zoarium discoid in the younger stages, becoming hemispheric or 
subconical with age; occasionally the centre part of the upper sur- 
face is drawn out, and in a few instances has been observed even to 
divide into two branch-like lobes. The last conditions, as well as 
various other irregular developments, are to be considered as abnor- 
mal. Base more or less concave, usually with a central cicatrix of 
attachment, beyond which it is covered with a concentrically-striated 
and wrinkled epitheca. Upper surface celluliferous. Height of 
zoarium varying from 5 mm. or less to 50 mm. or more; diameter 
from 10 to over 100 mm. Zoccia with direct, subcircular aper- 
tures, thin walls, those of neighboring cells generally in contact, ex- 
cept at the angles of junction, the latter being occupied by angular 
mesopores of variable, though usually small, size. In the youngest 
specimens the zowcia are the roundest, and the interspaces, occupied 
by the mesopores, the widest, while in the oldest the opposite condi- 
tions prevail. More or less conspicuous clusters of cells of larger 
size than the average occur at intervals of nearly 4 mm., measuring 
from centre to centre. Between these cells the mesopores are com- 
monly more numerous than elsewhere, and in nearly all cases con- 
stitute aggregations of variable extent and substellate form. In many 
cases, chiefly old examples, the mesopores between the zocecia occupy- 
ing the intermacular spaces might be overlooked, although, as shown 
in thin sections, they are really numerous even there. Diameter of 
an ordinary zocecium about 0.25 mm., with an average of eleven in 
