148 



BULLETIN 77, UNITED STATES NATIONAL MUSEUM. 



Occurrence. — The American type of the species was obtained in the 

 Phylloporina beds of the Black River (Decorah) shales near Fountain, 

 Minnesota. A single fairly well preserved specimen has been noted 

 in the Wassalem beds (D3) at Uxnorm, Esthonia. (Cat. No. 57244, 

 U.S.N.M.) 



ARTHROSTYLUS OBLIQUUS Ulrich. 



Text fig. 71. 



Arthrostylus obliquus Ulrich, Journ. Cincinnati Soc. Nat. Hist., vol. 12, 1890, 

 p. 190, figs. 14 c, d; Geol. Nat. Hist. Surv. Minnesota, vol. 3, pt. 1, 1893, 

 p. 188, pi. 3, figs, 15, 16. 



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 ex- 

 tremity. Zooecia in three rows, occupying as many faces of the 

 segment, the fourth side with three longitudinal strise, and no 

 zooecia. Profile of a segment in an obverse or reverse view, wavy 

 on both sides; in a lateral view only on one side. 



Zooscial apertures small, oblique, the posterior margin very 

 prominent, aiTanged alternately in the three rows, with nine in 

 each, in 2.5 mm. A short ridge from the upper depressed edge of 

 each zooecial aperture is flanked on each side by the prolonged 

 lateral borders of the aperture. No ridge between the lateral and 

 central row of the zooecia. 



1) I rpj^g isolated joints of this small but neat bryozoan 



are so inconspicuous that they are very likely to be 

 overlooked in collections. The species is very similar 

 to the preceding Arthrostylus conjunctus Ulrich, but 

 upon close examination may be distinguished by the 

 absence of ridges between the rows of apertures, the 

 prominent lower border, and especially by the oblique 

 zooecial apertures. 



Occurrence. — ^Ulrich's types of the species were found 

 in the Stictoporella bed of the Black River (Decorah) 

 shales, at Minneapolis, Minnesota, where specimens 

 are rather rare. The species is also of uncommon 

 occurrence in the Echinospherites limestone (Cl) 



at Archangelski, on the Wolchow River, in the government of St. 



Petersburg (Cat. No. 57245, U.S.N.M.). 



Specimens of this species from American localities are in the 



collections of the British Museum. 



Fig. 71.— Abtheo- 

 styltjs obliqutts. 



a, SIDE VIEW OF 

 PORTION OF A SEG- 

 MENT, X18; h, 

 NONCELLULIPEE- 

 OUSSIDE, X18, OF 

 THE TIPPER POR- 

 TION OF A SEG- 

 MENT. Black 

 Rivee(Decorah) 

 SHALES, Minne- 

 apolis, Minne- 

 sota. (After 

 Ulrich.) 



