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 zooecial 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 G. 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 zooecial tubes. 



Formation and locality. — Trenton limestone at Chatfleld 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 Ceramoporidm. 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 

 FistuUporidce, 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 pr^mature'evolutiou of the fistuliporoid 



