BRYOZOA. 201 
Arthroclema.] 
Compared with other species A. armatum, of the Galena shales, has larger and 
much longer segments, six or seven rows of zocecia, and the lower border of their 
apertures prominently produced. A, striatum, which is associated and more abun- 
dant on the same slabs, differs in having longer and differently shaped seg- 
ments, smaller zocecial apertures, generally six cycles of them, the surface striation 
finer and without papillie, and in having the angles but little if at all produced at 
the upper extremity. A. pulchellum Billings, differs in a similar manner. 
Formation and locality—The detached segments of this species are rather plentiful on certain lay- 
ers of the limestone plates in the lower part of the Trenton shales exposed in the railroad cut near the 
State University at Minneapolis. 
Mus. Reg. No. 8115. 
ARTHROCLEMA ARMATUM Ulrich. 
PLATE II, FIGS. 8-11, 25 and 28-33, and PLATE IIT, FIG. 7. 
Arthroclema armatum Uricu, 1890. Jour. Cin. Soc. Nat. Hist., vol. xii, p. 194, fig. 19, a, b, c, and, 
d; not e, f, g, h.* 
Zoarium jointed, consisting of three sets of segments, primary, secondary and 
tertiary, the first and second sets readily separable, the second and third probably 
indistinguishable in many cases. 
Primary segments generally subcylindrical, often compressed and irregularly 
shaped, without regular longitudinal angles, the surface usually appearing as simply 
striated, with strong spines and zocecial apertures indistinct ; length unusually vari- 
able, depending upon the number of cycles of zocecia in the segment, 3.0 mm. with 
seven cycles, 3.3 mm. with eight, 4.0 mm., the commonest size, with ten, and so on 
to 4.5 mm.; diameter varying with age from 0.5 to 1.0 mm.; ends truncate, the upper 
flat or raised centrally, the lower usually with a nipple-shaped prominence. Lateral 
articulating sockets deep, sharply defined, situated at or slightly above the middle 
of the length, so far as observed, never more than one to each segment; some are 
without any. Zocecia in six ranges, their apertures oval, small, slightly oblique, 
easily overlooked, sometimes, especially in the vicinity of the articulating socket, 
closed, with seven in about 2.5 mm.; width of lateral interspaces varying with age. 
A strong tubercle just behind or near each aperture. 
Secondary segments of about the same length as the primary set, usually about 
3.6 mm. They are, however, more slender, none being more than 0.5 mm. in diame- 
ter, ranging from that down to 0.3 mm. Upper end terminating abruptly, spinous ; 
lower end rounded. Lateral sockets faint, centrally situated, not often detected in 
this set. Zocecia rarely in five, commonly in six, occasionally in seven longitudinal 
*In the original work on this species one vertical section and three transverse sections of a Canadian example of 
A, pulchellum BILLINGS, were inadvertently given as A. armatum. These figures which are magnified x 25 instead of x 18, are 
reproduced in this volume on plate IT, 
