188 THE PALEONTOLOGY OF MINNESOTA. 



[Arthrostylus. 



longitudinal ridges ; the fourth face, commonly the widest, with longitudinal striae 

 only. 



Type : Arthronema tenue Ulrich (Helopora tenuis James), of the Iltica horizon of 

 the Cincinnati group. (Jour. Cin. Soc. 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. 



Aethrosttlus conjunctus Ulrich. 



PLATE III, FIGS. 13 and 14. 



Arthrostylus conjunctus Ulkich, 1890. Jour. Cin. Soc. Nat. Hist., yol. 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 striae. Zooecial 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. Eight zo'cecial apertures in each 

 row in 2.5 mm. 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 striae, causing transverse 

 sections to be more nearly square. The A. ohliguus differs in having oblique zocecial 

 apertures." 



Formation and locality.— ^a.Te near the base of the Trenton shales, near Fountain, Minnesota. In 

 the original description the locality is given, inadvertently, as Minneapolis. 



Artheostylus obliquus Ulrich. 



PLATE III, PIGS. 15 and 16. 

 Arthrostylus obliquus Ulkich, 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 



