American Paleozoic Bryozoa. 273 
in vertical sections, both being crossed by thick horizontal diaphragms, 
occurring at short and regular intervals, so as to divide the zoarium 
intoso many equal layers. The spiniform tubuli are numerous, of 
moderate size, and comparatively thin walled. Beside these a tan- 
gential section will show numerous, concentrically arranged, small 
dark or light spots, within the substance ot the horizontal partitions. 
I have been obliged to establish this genus for the reception of a 
most peculiar bryozoan from the Niagara group of Indiana, In its 
general form and superficial aspect, it does not present any striking 
differences from species of Atactoporella, except in the circular, and 
not petaloid zocecial. apertures. Thin sections, however, demonstrate, 
beyond a doubt, that the structural peculiarities of the two are widely 
different, and that, so far as our present knowledge extends, we have in 
Tdiotrypa an uniquely differentiated structural type. 
IpIoTRYPA PARASITICA, n. sp. (Pl. XHI., figs. 1-1c. ) 
Zoarium consisting of parasitic expansions, of variable thicknesses, 
usually attached to ramose bryozoa, but sometimes growing upon other 
foreign bodies. The true zocecia have circular apertures, often closed 
by centrally perforated opercula, of an average diameter of ;i,th of an 
inch, surrounded by a slightly elevated, thin, granulose, peristome. 
They are arranged with considerable regularity in series, in which 
from eight to ten may be counted in the space of :1 of an inch. The 
inter-tubular spaces are of somewhat variable thickness, and usually 
appear to be solid; occasionally, however, the mouths of shallow inter- 
stitial depressions may be observed. The spiniform tubuli usually do 
not constitute a conspicuous external feature. 
Tangential sections (PI. XIII, figs. 1 and 1c) show that the walls 
of the true cells are marked by a dark circular band, equivalent to the 
peristome surrounding the apertures. Within this band may be no- 
ticed, beside a limited number of rather thin-walled spiniform tubuli, 
a large number of smaller circular structures, usually provided like the 
spiniform tubuli, with a very minute lucid central spot. These doubt- 
lessly represent the granules above stated to occur on the peristome, in 
well preserved examples. The visceral cavities of all the cells appear to 
be more or less filled by sclerenchyma. This is due to the fact (shown in 
vertical sections) that the horizontal partitions are thicker at their june- 
tion with the walls of the tubes than they are more centrally, the cavities 
left between them being, therefore, of lenticular form, Some of the true 
