BRYOZOA. 177 



Artbropora.] 



The three species next following and A. shafleri (Meek) are the only species so 

 far published of which we know positively that they belong to Arthropora. There 

 are, however, at least three other distinguishable forms in the Cincinnati group of 

 Ohio, Indiana and Kentucky, each marking a particular horizon in the group. Most 

 of the species are abundant, but it is exceedingly rare to find any number of the seg- 

 ments still joined together, or lying in their original order. 



ARTHROPOKA SIMPLEX Ulrich. 



PLATE XIV, FIGS. 12-21. 



Arthropora simplex ULKICH, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Surv. Minn., p. 65. 



Zoarium jointed, rarely found except as isolated segments. Normally developed, 

 the segments are narrow, more or less compressed, unbranched, straight stems, 

 rounded and solid at each end, with sharp edges and striated non-poriferous border ; 

 12 to 19 mm. long (average length about 18 mm.), 1.0 to 1.8 mm. wide, and always 

 less than 1.0 mm. in thickness. The basal or primary segment is irregularly 

 branched, and occasionally some of the succeeding segments are divided, but such 

 divisions are evidently abnormal. A single specimen preserves several joints in 

 their natural position. From this it appears that, as a rule, the upper extremity of 

 each segment articulated with two succeeding segments. Basal segments thickest, 

 sometimes nearly cylindrical, their superficial characters obscured, the peristomes 

 and interstitial ridges thickened and the zorecial apertures reduced in size through 

 age. In the younger segments, and most specimens are to be so classed, the 

 characters are as follows : zocecia very regularly arranged in transverse and 

 diagonally intersecting series, with five in 1 mm. transversely, and eleven or 

 twelve in 3 mm. diagonally ; twenty-four to twenty-six of the transverse rows 

 in 5 mm. longitudinally. Zorecial apertures elliptical, surrounded by a very 

 thin, granose peristome. The latter is easily overlooked, strongly depressed at the 

 sides, but elevated and prolonged at each end, in most cases not far enough to con- 

 nect succeeding apertures ; separating the longitudinal rows an elevated, thin, papil- 

 lose, wavy ridge. In passing around the zooecial apertures these ridges alternately 

 diverge and converge, two coming close together, often even uniting, in the spaces 

 between the sides of the apertures. In many segments only the raised ends of the 

 inner depressed ring of papillae are distinguishable. In these cases the longitudinal 

 ridges combine in front and behind the apertures so as to produce an appearance 

 similar to fig. 22, plate XIV. 



