BRYOZOA, 191 
Helopora. } 
we do know that the zoarial combination of the segments in Arthroclema 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 granting 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 reeommended 
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 zocecial 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. 
Hetopora pivaricata Ulyich. 
PLATE III, FIGS. 1-3. 
Helepora divaricata ULRICH, 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 zocecial apertures six, seven or eight. Their diameter varies with age and 
according to the number of zocecia contained from 0.5 to 0.9mm. Zoccial 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 zocecial 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 
