BEYOZOA. 187 



Arthrostylus.l 



sidered tubular in the sense attaching to that term when applied to the Cyclostomata. 

 On the contrary they are no more so than are the zooecia of the most typical Rhinidic- 

 tyonidce. In tangential sections they are oblong-quadrate or hexagonal in outline. 

 Hemisepta have not been observed, but rows of minute tubuli occur between the 

 walls of adjoining zooecia and sometimes in the longitudinal interspaces in the ves- 

 tibular region. A minute tube is also to be detected running from end to end through 

 the axis of the segments. 



Respecting the position of the family the jointed character of the zoaria leads 

 us to look for its relationship first with the similarly constructed Cellariidce. The 

 latter embraces living forms chiefly, and of many of these I have secured specimens, 

 so that I am now in a position to speak intelligently upon their characters, as com- 

 pared with paleozoic forms. I would be glad to do this here were it not that I would 

 thereby interfere with my plans for a general work on inter-relation of bryozoan 

 types. It will probably be sufficient to say that the two families are distinguishable, 

 and that the relationship between them, if any exists, is almost certainly less inti- 

 mate than that between the Arthrostylidce and Rhinidictyonidce. 



Aside from the wedge-shaped form of the primitive cells in the Arthrostylidce, 

 ,which it is evident resulted necessarily from their radial arrangement about a linear 

 central axis, they are precisely like those of true Rhinidictya. They agree also in 

 possessing median tubuli between the walls of adjoining zooecia. Then again I am 

 convinced that the minute axial tube of the Arthrostylidce is functionally identical 

 with the median tubuli between the mesial laminae that are such a characteristic 

 mark of the Rhinidictyonidce. The jointed character of the zoarium even, is not 

 unknown in the latter since it pertains to Dicranopora, Ulrich, a genus that in all 

 other respects is precisely like Rhinidictya. Really, I find only one structural differ- 

 ence between the two families, and that is that while the zooecia in the Rhinidicty- 

 onidce are arranged so as to form bifoliate zoaria — in other words, are disposed in 

 two equal expansions grown together back to back, they are arranged radially 

 around a central axis, forming subcylindrical zoaria, in the Arthrostylidce. 



Genus ARTHROSTYLUS, Ulrich. 



Arthronema, Ulrich, 1882. Jour. Oin. Soc. Nat. Hist., vol. v, pp. 157 and 160 (not Eschscholtz, 



who used the name for a genus of Colpodea). 



Arthrostylus, Ulbich, 1888. Amer. Geologist, vol. i, p. 230; 1890, Jour. Cin. Soc. Nat. Hist., vol, xii, 



p. 188 ; 1890, 111. Geol. Sur., vol. viii, p. 400. 



Zoaria .bushy, branching dichotomously, the whole consisting of numerous, 

 exceedingly slender, subquadrate, equal segments, joined to each other by terminal 

 articulation. Zooecia arranged in three (perhaps more) rows, usually between 



