200 THE PALEONTOLOGY OF MINNESOTA. 



Arthroclema 



This species is readily distinguished from its associate, A. cornutum, by its longer 

 and differently shaped segments, narrower zooecial 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 zooecial 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. 



AKTHROCLEMA CORNUTUM Ulrich. 



PLATE II, FIGS. 16-21 and 23. 



Arthroclema cornutum ULBICH, 1890. Jour. Gin. 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 oh 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. Zooecia 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 ot segments, 

 peristomes of oval zooacial apertures, and the longitudinal interspaces more or less 

 regularly papillose. With age the number of rows of papillae 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 6.2 mm. 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 description I say "six-sided", but this proves to be an error. 



