232 



THE PALEONTOLOGY OF MINNESOTA. 



[Homotrypella mundula. 



Internal characters : Vertical sections shaw that the peripheral region is narrow, 

 that here, and in the turn of the zocecial tubes, the diaphragms are closely arranged, 

 with from six to eight in 0.5 mm. Just in the turn of the tubes, along the upper wall, 

 one, two or three cystiphragms were developed. The mesopores begin at the same 

 point, and in these the diaphragms are more crowded and thicker. In the central 

 part of the axial region the tubes are larger than farther out, and intersected by 

 diaphragms at intervals of about 0.5 mm. In nearing the surface the tubes become 

 narrower and the diaphragms closer. Tangential sections are distinctive in showing 

 the wide interpaces, large and sharply-deftned mesopores, and the ring-like defi- 

 nition of the zocecial walls. In most cases the zocecia are completely separated or 

 rarely touch each other without sacrificing any of their roundness of outline. 



Compared with other species, the^Homotrypa ? intercalaris of this work seems 

 to be the nearest and distinguished chiefly in vertical sections, diaphragms being 

 absent in the axial region of that species and cystiphragms more numerous in the 

 peripheral. For further remarks on this relationship, see p. 216. For comparisons 

 with H. mundula, see under that description. 



Formation and locality. — Galena shales, near Cannon Falls, Minnesota. Two fragments from the 

 middle Trenton shales at Minneapolis seem te be identical in nearly every respect, but so far the species 

 has not been found in the upper third of the shales, intervening between these two horizons. 



Mm. Beg. No. 8124. 



Homotrypella mundula, n. sp. 



Fig. 12. Homotrypella mundula Ulrich, Galena shales, Decorah, Iowa. 



a, tangential section; b and c, opposite sides of a vertical section, showing diflerences in the number 

 of cystiphragms; all x 18. 



Zoarium dendroid, branches subcylindrical, small, 2.5 to 5.0 mm. in diameter, 

 dividing, generally dichotomously, at irregular intervals. Monticules wanting, but 

 in most cases clusters of zocecia, slightly larger and more widerly separated than the 

 average, are to be distinguished. Zocecial apertures somewhat irregularly distributed, 



