BRYOZOA. 825 
Bythotrypa laxata.] 
type—that became extinct or was reabsorbed into the parent stock, failing to estab- 
lish a permanent line of development. In that case Crepipora? epidermata Ulrich, 
from the Hudson River rocks of Illinois, would seem to be the earliest known point 
in the direct line to Fistulipora. 
BytrHotrypPa LAXATA Ulrich. 
PLATE XXVIII, FIGS. 21-25. 
Fistulipora ? lawata ULRICH, 1889. Contri. Micro-Pal. Cambro-Sil. Rocks, Can., pt. ii, p. 37. 
Zoarium irregularly massive, usually beginning its growth upon some foreign 
body, the exposed under side strongly wrinkled and covered with an epithecal mem- 
brane. Specimens vary greatly in size, the smallest seen being about 12 mm. in 
diameter and 5 mm. or less high, while the Jargest is an oval mass 150 mm. long, 
120 mm. wide, and about 70 mm. high. In the lower third of the Trenton shales 
they are all small, none observed exceeding 50 mm. in diameter. In the middle 
and upper thirds specimens between 75 and 100 mm. wide are not rare, but masses 
exceeding that size have been met with only in the upper part of the Galena shales. 
Zocecial apertures subovate, nearly equal, direct or a little oblique, the lunarium 
broad, sharply elevated, sometimes seeming to arch slightly over the aperture; their 
arrangement appearing more irregular than it is, with nine or ten in 5mm. Meso- 
pores abundant, varying greatly in size, a few quite as large as the zocecia fronr 
which they are distinguished by their more angular and irregular form and in being 
without a lunarium. Mesopores forming larger or smaller clusters at irregular 
intervals from which the zoccial apertures are turned ina radial manner. These 
clusters are most inconspicuous—even difficult to make out under the glass—except 
under certain conditions of weathering when they stand out as subsolid spots. 
Under ordinary circumstances the whole surface seems to be occupied uniformly 
by an irregular network of cells. 
Internal characters: In transverse sections the appearance, aside from an unusual 
irregularity and looseness of arrangement, is much as in species of Fistulipora. The 
zocecia are irregularly pyriform or ovate in cross section, and have thin walls. The 
lunarium, though often not a very marked feature, is still always determinable by 
the more regularly curved semi-circular form of the lunarial side of the circumfer- 
ence of the zocwcium, the opposite side being, if not angular, at any rate always 
drawn to a circle of greater diameter than the lunarial side. Occasionally one or 
both ends of the lunarium may project into the zocecial cavity (see fig. 25). Not 
infrequently also the lunarial side is thickened by a light-colored deposit upon the 
