188 THE PALEONTOLOGY OF MINNESOTA. 
[Arthrostylus. 
longitudinal ridges; the fourth face, commonly the widest, with longitudinal stricze 
only. 
Type: Arthronema tenue Ulrich (Helopora tenuis James), of the Utica horizon of 
the Cincinnati group. (Jour, Cin. Soe. Nat. Hist., vol. v. p. 160, 1882.) The lower 
fig. 16 on Plate III, represents a transverse section of this species. It should have 
been been numbered 16a. 
Besides the type species only three others are known to have the characters 
demanded by this genus. ‘Two of these are from the Trenton shales of Minnesota, 
and are next described. The third species, A. curtus Ulrich, from the Hudson River 
rocks of Ohio, is still a little doubtful, no further material having been found to 
throw light upon the rather imperfect originals of the species. 
ARTHROSTYLUS consuNoTUS Ulrich. 
PLATE III, FIGS. 13 and 14. 
Arthrostylus conjunctus ULRICH, 1890. Jour. Cin. Soc. Nat. Hist., vol. xii, p. 189. 
Original description. “Zoarium jointed ; segments very slender, straight, needle- 
shaped, 3 or 4 mm. long, quadrangular in cross-section, 0.25 mm. wide, 0.18 mm. thick, 
with zocecial openings on three sides, the fourth being without them, but marked 
instead with four parallel longitudinal striz. Zocecial apertures broad-oval, direct, 
0.11 mm. long, 0.09 mm. wide, enclosed by a sharply marked peristome. Peristomes 
of each row of apertures joined together by a thin ridge, having a length about equal 
to the larger or outer diameter of the peristomes. Hight zocecial apertures in each 
row in2.5mm, A thin ridge on each side of the range of apertures of the obverse 
face of the segment separates it from the lateral rows. Apertures usually arranged 
alternately in the three rows. 
“This species is closely related to A. tenuis James, sp., but is distinguished by 
having the non-celliferous side narrower and with fewer striw, causing transverse 
sections to be more nearly square. The A. obliquus differs in having oblique zocecial 
apertures.” 
Formation and locality—Rare near the base of the Trenton shales, near Fountain, Minnesota. In 
the original description the locality is given, inadvertently, as Minneapolis. 
ARTHROSTYLUS OBLIQUUS Ulrich. 
PLATE III, FIGS. 15 and 16. 
Arthrostylus obliquus ULricu, 1890. Jour. Cin. Soc. Nat. Hist., vol. xii, p. 190. 
Original description: “Zoarium jointed, segments very slender, needle-shaped, 
straight or slightly curved, about 4 mm long, subquadrangular in cross-section, 0.2 
mm wide, 0.15 mm. thick, slightly expanding toward the upper extremity. Zocecia 
