200 THE PALEONTOLOGY OF MINNESOTA. 
Arthroclema 
This species is readily distinguished from its associate, A. cornutum, by its longer 
and differently shaped segments, narrower zocecial apertures and simply striated 
surface. It is closely allied to A, pulchellum Billings, but that species differs in hav- 
ing the segments a trifle shorter (eight or nine in 20 mm.), the articulating sockets 
situated near the middle instead of near the bottom of the joints, the primary seg- 
ments less expanded at the ends, the zocecial apertures more direct, larger and sepa- 
rated lengthwise by shorter interspaces. 
Formation and locality.—The detached segments of this species are very plentiful on some of the 
thin plates of limestone found in the lower part of the Trenton shales at Minneapolis and St. Paul, 
Minnesota. 
Mus. Reg. No. 8114. 
ARTHROCLEMA corNuTUM Ulrich. 
PLATE II, FIGS. 16-21 and 23. 
Arthroclema cornutum Uuricu, 1890. Jour. Cin. Soc. Nat. Hist., vol. xii, p. 193. 
Zoarium jointed, the segments five-sided, consisting possibly of three sets, but 
only two are positively known. For the sake of convenient reference these will be 
termed primary and secondary. 
Primary segments five-sided,* about 2 mm. long, 0.5 to 0.7 mm. in diameter, 
with the five angles more or less well-defined, though never sharp, produced at the 
truncated and hollow upper extremity into as many horn-like projections. Near the 
lower extremity, which is often a little bulbous and radially striated, there is usually 
one rather shallow, subcircular articulating socket. It would appear that the 
sockets were placed in these alternately on opposite sides of succeeding segments. 
There may have been, as I believed, originally two sockets on some of the segments, 
but no more than one is to be seen on any of those that have since been freed from 
the matrix. Zocecia in five longitudinal rows, and generally in four, rarely in five, 
cycles, the apertures of those in the uppermost cycle more oblique than the others 
and situated very near the upper extremity of the segment. Angles of segments, 
peristomes of oval zocecial apertures, and the longitudinal interspaces more or less 
regularly papillose. With age the number of rows of papille at the angles increases 
from one to three or four. 
Secondary segments also five-sided, 0.4 to 0.5 mm. in diameter, from 1.0 to 
1.8 mm. long, the usual length about 1.2 mm., with five in about 6.2mm. The length 
depends upon the number of cycles of zocecial apertures. These are commonly only 
three, but four cycles are not infrequent. Lateral articulating sockets apparently 
wanting in this set. Otherwise very similar to the primary series. The papillose 
ornamentation of the surface however is generally more regular, 
*In the original deseription I say “six-sided”, but this proves to be an error, 
