EAELY PALEOZOIC BRYOZOA OF THE BALTIC PROVINCES. 119 



centrica in the Borkliolm limestone, is also indicative of the fact that 

 the presence of Phsenopora ensiformis would not be unexpected. 



The zoarium is a narrow bifoliate expansion pointed at the base 

 for articulation and averaging 25 mm. in length and 2 mm. in Avidth; 

 each face bears from seven to eleven parallel rows of zocecia with six 

 zooecia in 2 mm. measuring longitudinally; the apertures are ar- 

 ranged in linear series between slightly raised longitudinal lines, 

 are quadrangular in shape, and measure about 0.18 by 0.30 mm. 

 In the marginal series they are oval, slightly oblique, and somewhat 

 larger than in the middle rows. The characteristic two mesopores 

 occupying the inner spaces between the ends of the apertures are 

 often discerned with difficulty, but upon close examination or by 

 use of thin sections the mesopores may be found in the position 

 normal for PhsBnopora. 



Occurrence. — Abundant in the Clinton rocks of western New York 

 and Ontario and not uncommon in the Rochester shale of the same 

 region. Rare in the Clinton rocks of Ohio and also in the Osgood 

 beds at Osgood, Indiana. "VYiman obtained the species from drift 

 bowlders of the Borkholm formation (F2) at Ojle Myr, island of 

 Gothland. 



Specimens from American localities are in the collections of the 

 United States National Museum and the British Museum. 



Genus ARTHROPORA Ulrich. 



Arthropora Ulrich, Journ Cincinnati Soc. Nat. Hist., vol. 5, 1882, pp. 152, 167. — 

 Miller, North Amer. Geol. and Pal., 1889, p. 293.— Ulrich, Geol. Surv. 

 Illinois, vol. 8, 1890, p. 393; Geol. and Nat. Hist. Surv. Minnesota, vol. 3, 

 pt. 1, 1893, p. 176.— PocTA, Syst. Sil. Center Boheme, vol. 8, pt. 2, 1894, 

 p. 14.— Ulrich, Zittel's Textbook of Paleontology (Eng. ed.), 1896, p. 279.— 

 Simpson, Fourteenth Ann. Rep. State Geologist of New York for the Year 

 1894, 1897, p. 605.— NiCKLES and Bassler, Bull. 173, U. S. Geol. Surv., 

 1900, p. 46. — CuMiNGS, Thirty-second Ann. Rep. Dep. Geol. Nat. Res. 

 Indiana, 1907, p. 739. 



Zoarium bushy, composed of numerous articulating equal seg- 

 ments, spread in a plane; apertures elliptical, surrounded by a deli- 

 cate peristome; interspaces with one or more thread-like ridges 

 variously disposed and with a row of minute papillae. 



Genotype. — Stictopora (Ptilodictya) shafferi Meek. Upper Ordovi- 

 cian (Maysville) of the Ohio Valley. 



The members of this interesting genus are readily distinguished 

 from all other species of the family by their segmented character. 

 Although complete zoaria are rare, their disjointed segments are 

 readily identified by the rounded and pointed ends adapted for 

 articulation. The genus is rather numerously represented in Amer- 

 ican Ordovician strata, but in Russia only a single species has been 

 noted. 



