Arthroc-ema] • BRYOZOA. 197 



Compared with species of Helopora, only two, H. spiniformis and H. mucronata, 

 require mention. Both have larger segments and the lower extremity more acute. 

 In the first the ridges and superficial striations are also more conspicuous, while in 

 the second the segments are curved and taper downward. 



Formation and locality.— GdA^na. shales, near Cannon Falls, Minnesota. 

 Genus ARTHROCLEMA, Billings. 



Arthroelema, Billings, 1862, Pal. Foss., vol. i, p. 54; Ulrich, 1886, Fourteenth Ann. Kep. Geol. 



Nat. Hist. Sur. Minn., p. 60; 1888, The American GeoloRist, vol. i, 

 p. 232; 1890, Jour. Cin. Soc. Nat. Hist., vol. xii, p. 192, and Geol. 

 Sur. 111., vol. viii, p. 400. 



Arthroelema (part), Ulkich, 1882. Jour. Cin. Soc. Nat. Hist., vol. v, p. 151. 



Zoarium jointed, composed of numerous subcylindrical segments, celluliferous 

 on all sides, arranged in a pinnate manner ; articulation both terminal and lateral. 

 Segments of three kinds, primary, secondary and tertiary. The first set forms the 

 strong central stem, of which each part has normally one or two sockets on opposite 

 sides for articulation with the smaller segments of the second set. The latter gen- 

 erally articulate in like manner, terminally with each other and laterally with the 

 still more slender segments of the third set. Zocecia subtubular, each occasionally 

 with a diaphragm, their apertures ovate, oblique, the inferior border more or less 

 prominent, arranged in rows between longitudinal ridges. Interspaces usually stri- 

 ated, often grano-striate. 



Type : A pulchellum Billings. 



Complete zoaria are known of only two species of this genus, the type and 

 A. billingsi. This is unfortunate because of the difficulty of determining the range 

 of variation that may obtain in the three sets of segments. If the student will 

 examine fig. 7, on plate II, he may appreciate the* difficulties referred to. This 

 illustration represents the fine type specimen of A. billingsi, and gives a good idea of 

 the arrangement and the differences in the size and length of the segments of the 

 three sets that may occur in a species of Arthroelema. As shown in the figure the 

 segments of the same set even may not be of uniform length. In A. billingsi the 

 first of the secondary set is twice the length of those succeeding it. Then again it 

 shows that while some of the secondary segments may articulate laterally with one 

 of the tertiary set, many others may be without them. As all the zoarial parts 

 spread approximately in the same plane, the development of tertiary segments must 

 depend very largely upon the space available. In A. billingsi this was much less 

 than in the other species since it is the only one known in which the primary seg- 

 ments articulate with four (two on each side) secondary joints. In all the other 



