BRYOZOA. 177 
Arthropora.] 
The three species next following and A. shafferi (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 ség- 
ments still joined together, or lying in their original order. 
ARTHROPORA SIMPLEX Ulrich. 
PLATE XIV, FIGS. 12-21. 
Arthropora simplex ULRICH, 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 zocecial apertures reduced in size through 
age. In the younger segments, and most specimens are to be so classed, the 
characters are as follows: zoccia 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. Zocecial 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 zocwcial 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 papillee 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. 
=! 
