206 THE PALEONTOLOGY OF MINNESOTA. 



[Nematopora. 



ora like R. lineinodis Ulrich, and E. regularis (Trematopora and later Orthopora regu- 

 laris Hall), but it is to be doubted that this resemblance is indicative of even remote 

 relation. In Rhombopora the primitive cells are drawn out into long tubes which 

 originate, just as they do in nearly all of the ramose Trepostomata, in various parts of 

 the axial region. In Nematopora, on the other hand, the zocecia are comparatively 

 short and arise along a definite axial line. 



Formation and locality. Galena shales, near Cannon Falls, Minnesota. 

 Mus. Reg. No. 8111. 



NEMATOPORA DBLICATULA Ulrich. 



PLATE III, PIGS. 26 and 27. 

 Nematopora delic.atula ULBICH, 1890. Geol. Sur. 111., vol. viii, p. 646. 



Zoarium very small and exceedingly slender, ramose above the pointed basal 

 extremity. Branches quadrangular, with the angles rather sharp, and the solid 

 sides flat or gently convex, each from 0.15 to 0.18 mm. wide. Zocecia rather long, in 

 four ranges ; apertures ovate or subcircular, about 0.12 mm. in length, separated by 

 long, smooth intervals, each from 0.7 to 1.0 mm. in length ; average length of zocecia 

 about 0.95 mm. Peristome thin, easily broken away ; when perfect it forms a 

 sharply elevated rim about each aperture. 



The widely separated zocecial apertures, and the exceeding delicacy and quad- 

 rangular form of the zoarium of this species distinguish it from all other species of 

 the genus known. The associated N. ovalis often has only four ranges of zocecia, but 

 their apertures are so much larger that the two species cannot be confounded. The 

 possibility of confusion with N. granosa and N. conferta is equally remote. Indeed, 

 all of these four associated species are distinguished by sharply marked and easily 

 recognized peculiarities. 



Formation and locality. Galena shales, near Cannon Falls, Minnesota. The original types of the 

 species were collected from an equivalent horizon in Alexander county, Illinois.* 



NEMATOPORA CONFERTA Ulrich. 



PLATE III, FIGS. 21-23. 

 / 



Nematopora conferta ULRICH, 1890. Jour. Gin. Soc. Nat. Hist., vol. xii, p. 198. 



Zoarium ramose, spreading nearly in a plane ; branches dividing dichotomously 

 at intervals varying from 1 to 3 mm., 0.4 to 0.5 mm. in diameter, subcircular in cross- 

 section. Zocecia in ftve or six longitudinal ranges, their apertures frequently arranged 



In the 111. Geol. Surv. vol. vlll, this horizon is given, in conformity with the preceding volumes of publications of 

 that survey, as Cincinnati group, but at the top of p. 645 (op. ci(.) it will be seen that I express a doubt as to their exact 

 age, saying that "I am inclined to regard them as more likely representing an upper member of the Trenton group." I 

 have now satisfied myself that they are equivalent to the Galena of the Northwest and the Trenton limestone of New York. 



