EARLY PALEOZOIC BEYOZOA OF THE BALTIC PROVINCES. 147 



Genus ARTHROSTYLUS Ulrieh. 



J.rt/ironemc! (preoccupied) Ulrich, Journ. Cincinnati Soc. Nat. Hist., vol. 5, 1882, 

 pp. 151, 160. 



Arthrostylus Ulrich, American Geologist, vol. 1, 1888,?p. 230; Journ. Cincinnati 

 Soc. Nat. Hist., vol. 12, 1890, p. 188; Geol. Surv. Illinios, vol. 8, 1890, p. 400; 

 Geol. and Nat. Hist. Surv. Minnesota, vol. 3, pt. 1, 1893, p. 187; Zittel's 

 Textbook of Paleontology, (Eng. ed.), 1896, p. 280. — Simpson, Fourteentli 

 Ann. Kep. State Geologist New York for the year 1894, 1897, p. 527.— 

 NiCKLES and Bassler, Bull. 173, U. S. Geol. Surv., 1900, p. 42. — Cumings, 

 Thirty-second Ann. Rep. Dep. Geol. Nat. Res. Indiana, 1907, p. 740. 



This interesting member of the Arthrostylidse is represented in the 

 Russian collections by a few individuals which can be referred to the 

 two American Black River species, A. conjundus and A. obliquus of 

 Ulrich. These specimens consist of free segments 

 of the zoarium, pointed at one end for articulation 

 with the preceding segment, and with a socket-like 

 hollow at the other end. The complete zoarium in 

 Arthrostylus consists of numerous very small sub- 

 quadrate segments united by terminal articulation 

 and branching dichotomously until a small pinnate 

 or bushy colony is the result; each of these seg- 

 ments has one of its faces longitudinally striated 

 and each of the other faces, of which there are com- 

 monly three, bears a linear series of apertures be- 

 tween longitudinal ridges. 



Genotype. — Helopora tenuis James. 

 Silurian (Richmond) of the United States 



Earliest 



ARTHROSTYLUS CONJTJNCTUS Ulrich. 



Text fig. 70. 



Fig. 70.— Arthrosty- 

 lus CONJUNCTUS. a, 

 LATERAL VIETV OF THE 

 CENTRAL PORTION OF 

 A SEGMENT, X18; 6, 

 SM.^LL PORTION OF 

 NONCELLU L I F E R O US 

 SIDE OF SAME, Xl8. 



Black River (Deco- 

 rah) shales, near 

 Fountain, Minne- 

 sota. (After Ul- 

 rich.) 



Arthrostylus conjundus Ulrich, Journ. Cincinnati Soc. Nat. 

 Hist., vol. 12, 1890, p. 189, fig. 14; Geol. and Nat. 

 Hist. Surv. Minnesota, vol. 3, pt. 1, 1893, p. 188, pi. 3, 

 figs. 13, 14. — Simpson, Fourteenth Ann. Rep. State 

 Geologist New York for the year 1894, 1897, p. 526, figs. 78, 79, 79a. 



This and the following species of Arthrostylus have been briefly but 

 clearly described by Ulrich in his memoir on the Bryozoa of Minnesota, 

 and, as the Russian examples referred to these two species differ in 

 no appreciable way, I have in each case made use of his description. 



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. Zocecial apertures broad-oval, direct, 

 0.11 mm. long, 0.09 mm. wide, inclosed 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 zocecial 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 alter- 

 nately in the three rows. 



