300 THE PALEONTOLOGY OF MINNESOTA. 



[Hemiphragma ottawense. 



rapidity. In the peripheral region, which is narrow and abruptly distinguished 

 from the axial, the walls are more or less thickened, and the tubes intersected by 

 semi-diaphragms, about four in 0.5 mm. I have satisfied myself that all the trans- 

 verse partitions in this outer part of the zocecial tubes are really incomplete. That 

 many may appear entire in sections is only because their inner edge happens to be 

 vertical instead of horizontal. Mesopores are difficult to make out in these sections, 

 being short and usually filled, in part at least, with solid tissue. Tangential sections 

 require no description, all the essential characters being shown in figs. 6, 7, 9 and 14. 

 In the axial part of transverse sections the tubes are unusually irregular and their 

 walls comparatively thick. 



This is a common and well marked species, but proves to be nearer H. ottawense 

 Foord sp., than I thought at first. Perhaps the only reliable difference, and that 

 may in part be due to the greater size of Foord's species, is the much smaller* num- 

 ber of diaphragms in H. irrasum. The largest specimens of the latter even do not 

 approach H. ottawense in the width of the closely tabulated peripheral region. 



Formation and locality. Common in the lower third and rare in the middle and upper thirds of 

 the Trenton shales at Minneapolis, St. Paul, and Preston. In the overlying Galena shales it is 

 again common at St. Paul and at various localities in Goodhue county. Also at Decorah, Iowa. 



Mus. Reg. Nos. 7618, 7627, 8033, 8041, 8051, 8078. 



HEMIPHRAGMA OTTAWENSE Foord. 



PLATE XXIV? FIGS. 1-4. 



Batestoma ottawense FOORD, 1883. Contri. to Micro-Pal. Cambro-Sil. Rocks, Can., p, 18. 



This species is so much like the preceding that it is sufficient to merely point 

 out the differences between them. In the first place H. ottawense is always of larger 

 size than H. irrasum, the width of its branches, which in some cases are strongly 

 compressed, varying -from 9 to over 30 mm., and their thickness from 8 to 12 mm. 

 Internally, the axial region is comparatively smaller and the tubes here have dia- 

 phragms which, though sometimes wanting for short distances, are yet more abund- 

 ant throughout the region than in H. irrasum. In the latter the peripheral region 

 is narrow and the number of semi-diaphragms correspondingly limited. In the 

 present species, however, this region is wide and the number of cross-partitions 

 often great. The difference in these respects is generally quite as marked as in 

 figures 3 and 5 on plate XXIV. 



Foord says of the Canadian form of the species (loc. cit.) that the surface exhibits 

 "small and inconspicuous monticules placed at variable distances apart and occu- 

 pied by from ten to fifteen cells slightly larger than the average." The monticules 



