198 TPTE PALEONTOLOGY OP MINNESOTA. 



[Artbroolema . 



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 which am inclined to identify a number of segments found by me in the 

 upper division o. Ike shales at St. Paul and Cannon Falls. A. billingsi is from a simi- 

 lar (perhaps a li.tle 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, 111. Geol. 

 Surv., from the upper beds at Wilmington, 111. It occurs also at Stony Mountain in 

 Manitoba. 



The relations of Arthroclema to Helopora have already been discussed in the 

 remarks on the latter genus. 



ARTHROCLEMA STRIATUM, n. 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. 28 ; (2) hundreds like 

 figs. 22 and 23, and (3) 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 



