American Paleozoic Bryozoa. DAT IT | 
_ward, and form what is called a vestibule, at the bottom of which is_ 
the true zoccial aperture. ‘This varies somewhat in size, and is often 
covered by a centrally perforated operculum. Interstitial cells and 
spiniform tubuli wanting. | 
Tangential sections (PI. XIIL., fig. 50) show that the zocecia have 
thick walls, and that the line of demarcation between them is sharp 
and distinct. The structure of the large (?) ocecial cells does not 
differ in any appreciable manner from that 0: the ordinary cells. 
Longitudinal sections (Pl. XIII., fig. 5c) show that at first the tubes 
are inclined, and separated by very thin walls. At the termination of 
this, the “immature” region, they assume an erect position, their 
walls are suddenly much thickened, and a diaphragm is thrown across 
the tube cavity. The walls now have a thickness equaling, or ex- 
ceeding the diameter of the true zocecial aperture, and are composed of 
a number of /\ shaped layers of sclerenchyma, each of which doubt. 
lessly marks a previous vestibule. The true apertures are often closed 
by opercula, which are usually preserved in place, in the tubes of the 
inner layer, when the zoarium consists of two layers. 
Formation and locality: Kaskaskia group. The species occurs at 
several localities in Kentucky, and, I believe, also in Illinois. The 
types are from Grayson Springs, and Tateville, Ky.; at the first place, 
specimens were not uncommon. 
HELIOTRYPA, 1. gen. 
Zoarium bifoliate. Cells of two kinds; true zocecia, with vestibules 
and subcircular orifices, and numerous smail iuterstitial cells. The 
prostrate, thin-walled, inner portion of the tubes, is divided from the 
outer, much thickened and erect portion, by an incomplete diaphragm, 
which springs from the convex wall at the termination of the prostrate 
or “immature” portion of the tube. In the peripheral or ‘* mature” 
region of the zoarium, the true zocecia are subcircular in form, more or 
less in contact, and provided with thick walls, which are perforated by 
numerous minute canals, that, on account of their oblique direction, 
give tangential sections a peculiar rayed appearance. The line of 
separation between all the cells is marked by a row of vertical tubuli, 
with which the oblique series seem to communicate. These again 
appear to be in connection with other canals, that traverse the in- 
tervals between the basal membranes of the two leaves, and the com- 
paratively thin walls of the tubes in the “immature” region of the 
_zoarium. The interstitial tubes are developed from the prostrate 
