BEYOZOA. 191 



Helopora.l 



we do know that the zoarial combination of the segments in Arthroclerm is by both 

 terminal and lateral articulation, while in Helopora they unite at their ends only. 

 The difficulties, therefore, which we may experience in correctly classifying some of 

 the dismembered segments, are not at all encountered when we deal with complete 

 zoaria. Even gi-anting that the latter condition is exceedingly rare, the trouble of 

 discriminating between the isolated segments of the two genera is not of very com- 

 mon occurrence, because it is restricted to those of the third order of Arthroclema, 

 those of the primary and secondary set being easily distinguished from Helopora by 

 the lateral articular socket. In practice I think we are nearly safe under this work- 

 ing rule : When of a number of isolated segments occurring on slabs of rock or in 

 the residue of shale washings, none have lateral sockets, it is safe to classify them as 

 Helopora, providing, of course, that they agree with that genus in other respects. 

 When, however, one or more of them possess such sockets, it is to be recommended 

 that the investigator determine the three sets of segments of the Arthroclema before 

 he classifies any of them as Helopora. , 



Of the remaining genera of the family, Sceptropora is distinguished by the wide 

 expansion or swelling of the upper half of the segments ; Arthrostylus in having one 

 side simply striated and without zooecial apertures, and Nematopora by its branch- 

 ing zoaria and the absence of joints above the basal articulation. The placing of 

 such species with Helopora, by Billings, was an error that I am convinced he would 

 not have committed had he known that Helopora originally consisted of numerous 

 subequal segments joined together into bushy zoaria. 



Helopora divaeicata Ulrich. 



PLATE III, FIGS. 1-3. 



Helopora divarieata Uleich, 1886. Fourteenth Ann. Rep. Geol. Nat. Hist. Sur. Minn., p. 59. 



Zoarium jointed ; segments about 7.0 mm. long, obtuse at both extremities, sub- 

 cylindrical, polygonal in cross-section, the number of the angles and corresponding 

 rows of zooecial apertures six, seven or eight. Their diameter varies with age and 

 according to the number of zooecia contained from 0.5 to 0.9 mm. Zooecial apertures 

 comparatively large, oblique, ovate, seeming to widen anteriorly, arranged in troughs 

 between strong longitudinal ridges, twelve in 5 mm. lengthwise and generally in 

 regular transverse rows. Posterior border of apertures thick, prominent, sloping 

 backward into the aperture next below. This border is continued upon the sides of* 

 the zooecial aperture as two diverging ridges which extend on each side to the sum- 

 mit of the longitudinal keels where they meet with similar ridges from the adjoin- 

 ing rows. These divaricating ridges cause the strong vertical keels to appear as 



