.^,, , , BRYOZOA. 201 



Arthroclema.l 



Compared with other species A. armatum, of the Galena shales, has larger and 

 much longer segments, six or seven rows of zooecia, 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 zooeciai apei-tures, generally six cycles of them, the surface striation 

 finer and without papillaB, 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.Beg. No. 8115. 



Arthroclema armatum XJlrich. 



PLATE II, FIGS. 8-11, 25 and 28-33, and PLATB*III, FIG. 7. 



Arthroolema armatum TJleich, 1890. Jour. Cin. Soc. Nat. Hist., vol. xii, p. 194, flg. 19, a, b, c, and, 



d; note, 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 zooeciai apertures indistinct ; length unusually vari- 

 able, depending upon the number of cycles of zooecia 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. Zooecia 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 

 8.6 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. Zooecia rarely in five, commonly in six, occasionally in seven longitudinal 



^*I the original work on this sjiecies one vertical section and three transverse sections of a Canadian example of 



ulchellum BiiiLiNOS, were Inadvertently eiven as A. armatum. These figures which are magnified x 25 Instead of x 18, are 

 reproduced in this volume on plate II. 



