BRYOZOA. ; 197 
Arthroc’ema.] 
Compared with species of Helopora, only two, H. spiniformis and H. mucronata, 
require mention. Both have larger segments and the lower extremity more acute. 
In the first the ridges and superficial striations are also more conspicuous, while in 
the second the segments are curved and taper downward. 
Formation and locality—Galena shales, near Cannon Falls, Minnesota. 
Genus ARTHROCLEMA, Billings. 
Arthroclema, BILLINGS, 1862, Pal. Foss., vol. i, p. 54; Untricn, 1886, Fourteenth Ann. Rep. Geol. 
Nat. Hist. Sur. Minn., p. 60; 1888, The American Geologist, vol. i, 
p. 232; 1890, Jour. Cin. Soc. Nat. Hist., vol. xii, p. 192, and Geol. 
Sur. Ill., vol. viii, p. 400. 
Arthroclema (part), ULRICH, 1882. Jour. Cin. Soc. Nat. Hist., vol. v, p. 151. 
Zoarium jointed, composed of numerous subeylindrical segments, celluliferous 
on all sides, arranged in a pinnate manner ; articulation both terminal and lateral. 
Segments of three kinds, primary, secondary and tertiary. The first set forms the 
strong central stem, of which each part has normally one or two sockets on opposite 
sides for articulation with the smaller segments of the second set. The latter gen- 
erally articulate in like manner, terminally with each other and laterally with the 
still more slender segments of the third set. Zocecia subtubular, each occasionally 
with a diaphragm, their apertures ovate, oblique, the inferior border more or less 
prominent, arranged in rows between longitudinal ridges, Interspaces usually stri- 
ated, often grano-striate. 
Type: A pulchellum Billings. 
Complete zoaria are known of only two species of this genus, the type and 
A, billingsi. This is unfortunate because of the difficulty of determining the range 
of variation that may obtain in the three sets of segments. If the student will 
examine fig. 7, on plate II, he may appreciate the difficulties referred to. This 
illustration represents the fine type specimen of A. billingsi, and gives a good idea of 
the arrangement and the differences in the size and length of the segments of the 
three sets that may occur in a species of Arthroclema. As shown in the figure the 
segments of the same set even may not be of uniform length. In A. billings: the 
first of the secondary set is twice the length of those succeeding it. Then again it 
shows that while some of the secondary segments may articulate laterally with one 
of the tertiary set, many others may be without them. As all the zoarial parts 
spread approximately in the same plane, the development of tertiary segments must 
depend very largely upon the space available. In A. billingsi this was much less 
than in the other species since it is the only one known in which the primary seg- 
ments articulate with four (two on each side) secondary joints, In all the other 
