236 THE PALEONTOLOGY OF MINNESO'^A. 



[Homotrypa exilis. 



walls and more or less oblique apertures. In young examples the obliquity is very 

 great, while it is only in the largest that the apertures can be said to be direct. About 

 twelve of the average zocecia in 3 mm. In the large monticulose variety, already men- 

 tioned, and which may be designated as var. montifera, there are thirteen apertures 

 in the same space. 



Internal characters: In vertical sections the peripheral or" mature" region is 

 very narrow, and the axial region, in which the tubes are long and nearly vertical, 

 correspondingly large. In the latter diaphragms are wanting, and the walls extremely 

 thin and wavy. Near the surface the walls are appreciably thickened, but never 

 enough to be described as otherwise than thin. The curve of the tubes throughout 

 is unusually gentle. Diaphragms and cystiphragms set in abruptly and their arrange- 

 ment in the tubes is shown better in figs. 1, 5 and 6 than it can be described. 



Owing to the obliquity ^of the zocecial apertures it is difficult to prepare satis-, 

 factory tangential sections of any except large and old examples. The successful 

 ones show that the zocecial walls are comparatively thin, that a few mesopore-like 

 cells are sometimes interpolated between the zocecia, and that the acanthopores are 

 so small and few that they are readily overlooked. Nor are the cystiphragms con- 

 spicuous in these sections. 



Transverse sections are interesting chiefly because they show the unusual nar- 

 rowness of the peripheral region, and the decrease in size and flattemng of the tubes 

 as they enter this part of the zoarium. 



This species is distinguished from all the others of the genus known from the 

 Trenton by its oblique zocecial apertures. 



Formation and locality.— Common in the lower third of the Trenton shales, at Minneapolis, St. 

 Paul, Cannon Falls, Preston, Fountain and other localities in the state. It has not been certainly identi- 

 fied in the middle third of the shales, but in the upper third, at St. Paul, a large form of this species occurs 

 associated with the var. montifera in considerable abundance. The species is also known from Decorah 

 and other localities in Iowa. 



Mus. Beg. »os. 5970, 5975, 7600. 



HoMOTKYPA EXILIS TJlrich. 



PLATE XIX, FIGS. 10-16. 



Homotrypa exilis Ulkich, 1886. Fourteenth Ann. Eep. Geol. Nat. Hist. Surv. Minn., p. 80. 



Zoarium ramose, branches slender, without monticules,,, cylindrical, 3 to 5 mm. 

 in diameter (generally 4 mm.), dividing at long intervals. Entire hight of zoarium 

 less than 75 mm. Zocecia with rounded, direct apertures and moderately thick 

 walls, about twelve in 3 mm. Clusters of cells larger than the average occur, but 

 do not constitute a conspicuous feature. Mesopores comparatively numerous, 



