244 THE PALEONTOLOGY OF MINNESOTA. 



[Prasopora. 



tubes from the series of cystiphragms to the opposite wall, are still preserved almost 

 to the surface. That they are often wanting, as shown in the figures, is probably due 

 to imperfect preservation. 



This species is clearly distinct from all the preceding. Its relations seem to be 

 with H. curvata of the Cincinnati group. Superficially it resembles Monticulipora 

 arborea Ulrich, a form that is associated with it not only in Minnesota but also in 

 Kentucky. That species is separated by its smaller cells and finely granulose walls. 

 Internally they are quite distinct. 



Formation and locality. Galena shales, near Cannon Falls, Minnesota ; shales of the Trenton group 

 at Burgin and Frankfort, Kentucky. 



Genus PRASOPORA, Nicholson and Ethridge, jun. 



Prasopora, NICHOLSON and ETIIIUDOE, jun., 1877, Ann. and Mag- Nat. Hist., ser. 4, vol. xx. p. 38; 



1879, NICHOLSON, Pal. Tab. Corals, p. 324, and 1881, "Genus Monti- 

 culipora," pp. 102 and 202; 1882, ULRICU, Jour. Gin. Soc. Nat. Hist., 

 vol. v, p. 153, and 1890, Geol. Surv. 111., vol. viii, p. 371; 1887, FOERSTJC, 

 Ball. Sci. Lab. Dennison Univ., vol. ii, p. 170. 



Zoaria forming conical, hemispheric, discoid or irregular masses ; at other 

 times growing in thin expansions ; usually free in the adult state, with a wrinkled 

 epithecal covering upon the lower concave or flat side. Zorecial tubes prismatic or 

 cylindrical, thin-walled, partially separated from each other, occasionally completely 

 isolated by smaller angular mesopores, best seen in young specimens and thin trans- 

 verse sections, and perhaps always smaller and less readily distinguishable at the 

 surface of mature or old examples. Acanthopores usually present, but in most cases 

 neither numerous nor strong. Cystiphragms in all the zocecial tubes. Diaphragms 

 crowded in the mesopores. 



Type : P. grayce Nicholson and Ethridge. 



This is one of the best characterized genera of paleozoic Bryozoa. The species 

 moreover are most of them common fossils and widely distributed, so that the genus 

 is also important in a geological sense. Most of the species are restricted to the 

 Trenton rocks, and only one, P. parmula Foerste, of the Clinton of Ohio, is as yet 

 known from strata above the top of the Lower Silurian, 



