234 THE PALEONTOLOGY OF MINNESOTA. 



[Homotrypella rustlca 



HOMOTRYPELLA BUSTICA, W. Sp. 

 PLATE XVIII, FIGS. 31-33. 



Zoarium irregularly ramose, branches 5 to 10 mm. in diameter. Low swellings 

 of the surface, scarcely to be called monticules, occasionally present. Surface very 

 rough under a hand lens, the acanthopores being strong and numerous, though not 

 materially inflecting the zooecial walls. Zooecial apertures rounded, about eleven in 

 3 mm. Mesopores abundant, though but rarely separating the zooe'cia completely, 

 of unequal sizes, rounded at the surface. 



Internal characters: In tangential sections, showing the characters immediately 

 beneath the surface, (upper part fig. 32) the zooecia are rounded, with only moder- 

 ately thick walls, the mesopores sharply defined, subangular, of unequal sizes, and 

 averaging three or four to each zooecium, the acanthopores strong, perhaps two to 

 each zooecium, and situated chieflly in the zooecial walls, which they occasionally 

 only cause to bend inwardly. At a deeper level (lower part fig. 32) the walls are 

 thinner, the acanthopores smaller, and the mesopores larger. At both levels the 

 zooecia almost uniformly exhibit the cut edges of cystiphragms. In vertical sections 

 the most striking feature of the species is the abundant tabulation of all the tubes. 

 Diaphragms occur all through the axial region, and both the mesopores and acantho- 

 pores began earlier than usual. The outward curving of the tubes also is unusually 

 gradual. The diaphragms in the two sets of tubes are subequally distributed, and 

 it is often difficult to discriminate between them when the curved edges of the 

 cystiphragn^s are not shown. At about the middle of the curve nine or ten dia- 

 phragms occur in 1 mm.; nearer the surface they are a little closer, while more toward 

 the center of the branch they are further apart. The cystiphragms are unusually 

 superficial in this species, forming crowded series almost to the mouths of the 

 zooecia. 



This species is distinguished from H. granulifera and H. mundula by the greater 

 number and open character of the mesopores ; from H. multiporata by its larger 

 zooecia, fewer and more unequal mesopores, and more crowded as well as different 

 tabulation of the tubes. H. instabilis has thicker walls, and is quite diflferent in other 

 respects. 



Formation and locality.— The sections illustrated were prepared from a siugle example collected by 

 the author in the upper beds of the Hudson River group, at Spring Valley, Minaesota. This specimen 

 seems identical with a common species occurriog at au equivaleat horizon iu Indiana and Ohio, to which 

 I had applied the name rustica in my MS. notes. 



Mus. Beg. No. 8125. 



