240 THE PALEONTOLOGY OF MINNESOTA. 



[Homotrypa tuberculuta 



their diameter ; that in the peripheral region the walls are moderately thickened, 

 and the tubes occupied by a gradually crowding series of cystiphragms. The varying 

 appearances of tangential sections are sufficiently exhibited in figs. 22 to 25. The 

 differences so far observed in these sections consist almost entirely of variations in 

 the number and size of the acanthopores. 



Greatly increased collections, and the study of numerous sets of thin sections, 

 have convinced me of the specific identity of H. subramosa and //. insignis. The 

 latter name might be retained in a subordinate sense for the Galena shales variety, 

 in which the clusters of large cells are more conspicuous, the average size of 

 the zoarium smaller, the zoo3cial walls thinner, and the acanthopores permanently 

 less numerous and smaller than in the typical middle and upper Trenton shales 

 form of the species. 



The tabulated axial region, more numerous and larger cystiphragms, the 

 presence of acanthpores, and the irregular and more compact growth, will distin- 

 guish the species from preceding forms of Homotrypa. Thin sections will of course 

 separate it at once from outwardly more similar species, belonging to other genera, 

 that are associated in the same beds. 



Formation and locality. H. subramosa is rare in the middle third of the Trenton shales at several 

 localities in St. Paul and Minneapolis, but is much more abundant in the upper third of the shales at St. 

 Paul and localities in Goodhue county, Minnesota. The restricted var. insignit is comparatively rare in 

 the Galena shales at localities in Goodhue and Fillmore counties; also in the same beds at St. Paul, and 

 at Decorah, Iowa. 



Mus. Reg. Nos. 5977 to 5980, 8030, 8061. 



HOMOTEYPA TUBERCULATA, H. Sp. 



FIG. 14 Homotrypa tuberculata, n. sp., upper third Trenton shales, near Cannon Falls, Minnesota. 



Collection of E. O. Ulrich. a, fragment of the natural size; b, small portion of a vertical section, x 18, 



showing two mesopores, the cystiphragms and diaphragms of the peripheral region, all of which are want- 



ng in the axial region; c and d, two portions of a tangential section of a fully matured example, x 18, the 



former showing appearance immediately beneath the surface, the latter at a slightly deeper level. 



