J 
198 THE PALEONTOLOGY OF MINNESOTA. 
[Artbroclema . 
species there is only one on each side, so that their growth was less compact, and 
the chances for tertiary segments better. Still, it is possible that the third set of 
joints may not, in some species, have been developed at all. This seems to be true 
of A. cornutum, since lateral sockets have not been observed on any of the numerous 
secondary segments of that species seen. 
Regarding the present work on the genus I regret that I have not been able to 
do the species justice in the way of illustration. I have sought to figure them as 
fully as the plates at my disposal admitted. But with so many conditions demand- 
ing representation, I was often at a loss in selecting the specimens which would go 
farthest in aiding the student to identify the species. 
Respecting the distribution of the species, A. cornutum and A. striatum are known 
as yet only from the middle third of the Trenton shales of Minnesota. The next 
species is the A. pulchellum Billings, described from the Trenton limestones of Canada, 
and with whick ~ vm inclined to identify a number of segments found by me in the 
upper division 0 ‘ue shales at St. Paul and Cannon Falls. A. billingsi is from a simi- 
lar (perhaps a litle lower) horizon at Ottawa, Canada. The detached segments of A. 
armatum were abundant in washings of the Galena shales obtained from a locality 
near Cannon Falls. The last species is the A. angulare, described in vol. viii, Hl. Geol. 
Surv., from the upper beds at Wilmington, Ill. It occurs also at Stony Mountain in 
Manitoba. 
The relations of Arthroclema to Helopora have already been discussed in the 
remarks on the latter gemus. 
ARTHROCLEMA STRIATUM, 7%. Sp. 
PLATE II, FIGS. 22 and 24, and PLATE III, FIGS, 28-33, 
Under this name I propose to arrange a variety of segments that, though closely 
related to A. pulchellum Billings, are evidently distinct. The specimens consist (1) of 
a relatively small number of segments like the original of fig. 25; (2) hundreds like 
figs. 22 and 28, and (8) equally numerous more slender segments. They agree with 
each other in what I regard as specific characters, and in the following description 
are referred to respectively as primary, secondary and tertiary segments. 
Primary segments 2.5 to 3.0 mm. long, from 0.7 to 1.0 mm. in diameter in the 
middle, spreading at the ends to a diameter of 1.2 mm. or more, the upper extremity 
flat, generally subcircular, the lower also abrupt but rising centrally into an obtusely 
conical elevation. Sometimes with only a single, large, deep and sharply defined 
socket, situated very near the base; more commonly with two, placed on opposite 
sides of the segment. Occasionally a third socket, this one small and illy defined, is 
