324 THE PALEONTOLOGY OF MINNESOTA. 
[Bythotrypa’ 
The appearance of thin sections is shown in the figures on plate XXVIII. In 
figs. 30 and 31 the presence of the lunarium is shown in an unmistakable manner, 
but fig. 32 is more like the usual appearance. Indeed, the lunarium is often so 
difficult to distinguish in transverse sections, that it is in order to caution the stu- 
dent against confusing the species with Monotrypa. In vertical sections the walls 
are often minutely crenulated, and in most cases exhibit the transverse lineation so 
common among the ceramoporoids. Exceedingly thin diaphragms occur in all the 
tubes at intervals varying from one to two tube-diameters. 
The great size of the zocecial tubes separates this species from all the other 
forms of Crepipora known. In this respect the species is approached by but one other 
paleozoic bryozoan, the Monotrypa magna of the present work, and both are believed 
to occur in nearly the same geological horizon. These two forms also present some 
points of resemblance in vertical sections, but so far as I can see there is really no 
relationship between them. In the Monotrypa the zocecia are much more regularly 
angular, their walls without the transverse lineation and more coarsely wavy, while 
a lunarium is of course never present. The C. hemispherica Ulrich, which seems to 
occur in the shales of the Hudson River group at Granger and near Spring Valley, 
differs chiefly in the smaller size of the zocecial tubes. 
Formation and locality—Trenton limestone at Chatfield and two miles northeast of Spring Valley, 
Minnesota. 
Mus, Reg. Nos. 151, 170, 211. 
Genus BYTHOTRYPA, n. gen. 
Zoaria massive or lamellate. Zocecia forming long continuous tubes, intersected 
by thin diaphragms, their walls minutely crenulate and with the structure charac- 
terizing the ceramoporoids. Lunarium well defined, large, projecting above the rest 
of the aperture margin. Mesopores numerous, open at the surface, interiorly form- 
ing a species of vesicular tissue unusually loose and irregular in construction. 
Type: Fistulipora laxata Ulrich. 
Largely increased collections of the type of this genus have convinced me that 
the species really belongs to the Ceramoporide. As none of the established genera 
of that family would include it, a new generic division became necessary. Bythotrypa 
is probably, as I regarded it at first, a type of structure that culminated in true 
Fistuliporide, but the lines along which the development progressed we are as yet 
unable to define. Still, it is more than possible that we have here merely a fore- 
shadowing of that family—in other words, a premature evolution of the fistuliporoid 
