304 THE PALEONTOLOGY OF MINNESOTA. 



[Monotrypa magna. 



The genus as restricted embraces but few species. Besides the type, which 

 occurs in the Trenton of Canada and New York, we have M. subglobosa Ulrich, in 

 the Utica horizon at Cincinnati, M. rectimuralis Ulrich, in the Trenton of southern 

 Illinois and probably in the Cincinnati group of Ohio, M. intabulata, n. sp., and M. 

 (? Chcetetes) cumulata, n. sp., in the Galena of Minnesota, M. magna, n. sp., in the 

 Birdseye limestone of the northwest, M. colliculata (Cfuctetes colliculatus Hall,) in the 

 Lower Helderberg of New York, and M. tabulata (Ptychonemn tabulatum Hall,) in the 

 Upper Helderberg of New York. A small undescribed species occurs in the Niagara 

 of Indiana, while another, apparently belonging here, I found in the Corniferous 

 limestone at Columbus, Ohio. 



MONOTRYPA MAGNA, n. sp. 



PLATE XXVII, FIGS. 28 and 29. 



Zoarium growing in large expanded masses, sometimes consisting of superposed 

 layers, the whole perhaps 20 to 40 mm. high and 100 mm. wide; under side generally 

 with a wrinkled epitheca, the upper celluliferous and without monticules. Zooecia 

 large, polygonal, thin-walled, with clusters of larger size than the average at inter- 

 vals of about 6 mm., measuring from center to center; about nine in 5 mm.; size of 

 largest in the clusters about 0.8 mm., average diameter of these in the spaces between 

 the clusters about 0.5 mm. 



Internal characters-' In vertical sections the zorecial walls are strongly undulat- 

 ing and very thin throughout, and the tubes crossed by complete horizontal dia- 

 phragms at intervals varying between one and three times the diameter of a tube. 

 Transverse sections exhibit thin structureless walls, an occasional small (young) 

 cell, and a total absence of mesopores and acanthopores. 



The larger size of the zocecia distinguishes the species from M. undulata Nichol- 

 son.. Excepting the Crepipora perampla of the present work, they are larger than in 

 any paleozoic bryozoan known to me. The zoaria of that species are comparatively 

 higher and less expanded, and their zooscia provided with lunaria. 



Formation and locality. Not uncommon in the "Lower Blue" limestone at Dixon, Illinois; also 

 at Mineral Point and Beloit in Wisconsin. A small fragment from the equivalent limestone at Minne- 

 apolis is provisionally identified with it. 



