258 THE PALEONTOLOGY OF MINNESOTA. 
(Mesotrypa. 
Mesorrypa mnripa Ulrich. 
PLATE XVII, FIGS. 1-8. 
Diplotrypa infida ULRICH, 1886, Fourteenth Ann. Rep. Geol. Nat. Hist. Sur. Minn., p. 88. 
Zoarium discoid or subhemispheric, with the base flat or concave and partly 
covered with a concentrically wrinkled epitheca ; or it may be parasitic and conform 
with the shape of the body grown upon; hight or thickness from 2 to 7 mm., diame- 
ter from 12 to 20mm. Zovcial apertures varying from polygonal to subcircular, 
the shape depending upon the number and size of the mesopores. In some speci- 
mens the latter are almost certain to be overlooked, the zoccia being angular and 
seemingly in perfect contiguity (pl. XVII, fig. 8). In others they are large enough 
to constitute an obvious external feature (fig. 7). The latter condition is to be 
regarded as less mature than the former, since in it the acanthopores are scarcely 
distinguishable, while they are readily made out where the mesopores are smallest. 
At intervals of 3 or 4 mm. there are clusters of large zocecia varying in diameter 
‘from the smaller or ordinary sizes of 0.2 to 0.24 mm. to 0.4 or 0.45 mm. An average 
of eleven of the ordinary size in 38 mm. 
Internal structure: In vertical sections the tubes are everywhere perpendicular 
to the basal membrance. Their walls are a little thicker than usual in species of 
the genus. In the lower part of the zoarium the mesopores with their numerous 
diaphragms are very conspicuous. In following them upward they seem to be re- 
duced in diameter, permitting the zocecial tubes to come in contact with each other. 
These zones may be repeated several times. Diaphragms are numerous though not 
regularly distributed in the zocecial tubes. The appearances presented by them are 
exceedingly variable. Some appear to be horizontal and straight or slightly bowed 
down in the middle (figs. 4 and 6), others are obliquely curved and perhaps overlap- 
ping (fig. 3), while still others are funnel-shaped (fig. 5). This variability however 
is not a structural peculiarity, but is due to the varying angles at which the dia- 
phragms are cut by the section. 
Tangential sections vary according to the depth from the surface at which they 
divide the zoarium. In the immature region (fig. 2) we have rounded zocecia, almost 
completely surrounded by mesopores, and small acanthopores. In the mature region 
(fig. 1) the mesopores are much smaller, the zocecia somewhat larger and subangular, 
and the acanthopores larger. In most of the zocecial cavities the oblique and some- 
what funnel-shaped diaphragms are represented by curved lines, often closely sim- 
ulating the appearances of ordinary cystiphragms. 
