BRYOZOA. 189 
Helopora. ] 
in three rows, occupying as many faces of the segment, the fourth side with three 
longitudinal striae, and no zocecia. Profile of a segment in an obverse or reverse 
view, wavy on both sides; in a lateral view only on one side. 
“Zocecial apertures small, oblique, the posterior margin very prominent, arranged 
alternately in the three rows, with nine in each, in 2.5mm. A short ridge from the 
upper depressed edge of each zowcial aperture is flanked on each side by the pro- 
longed lateral borders of the aperture. No ridge between the lateral and central 
row of the zocecia. 
“The oblique zocecial apertures, the prominent lower border and absence of ridges 
between the rows of apertures, distinguish this species from A, conjunctus and A. 
tenuis, both of which it resembles in other respects.” 
Formation and locality.—Trenton shales, Minneapolis, Minnesota ; rare. 
Genus HELOPORA, Hall. 
Helopora, HALL, 1852, Pal. N. Y.. vol. ii, p. 44; Brniinas, 1866, (part.) Cata. Sil. Foss. Isl. Antic., 
p. 836; ULRICH, 1888, The Amer. Geologist, vol. i. No. 4, p. 231, 1890, 
Jour, Cin. Soc. Nat. Hist., vol. xii, p. 191, and 1890, Geol. Surv. IIl., 
vol. viii, pp. 401 and 642, 
Zoaria consisting of numerous, subequal, small, cylindrical segments, articulating 
terminally, poriferous on all sides. Zocecial tubes somewhat oblique, geniculated or 
proceeding to the surface in a straight line. Apertures slightly oblique or appearing 
direct, suboval, arranged in diagonally intersecting series (section a) or between more 
or less well defined iongitudinal ridges (section 5). In section a the apertures are 
usually without a peristome, but an acanthopore occurs immediately beneath each. In 
section b the acanthopores are wanting, but a peristome, generally incomplete and 
prominently elevated posteriorly, is present. Axial tube very slender. 
Type: A. fragilis Hall, a common fossil of the Clinton group. 
As is indicated above, this genus may be divided into two sections. These were 
noted in my previous work on the genus (loc. cit.) andin one of them I express the 
opinion that, when these fossils are better understood, these two sections will 
probably be separated generically. Although the study of the genus, necessitated 
by the present work, has strengthened this opinion, I am not yet ready to make 
the separation. Still, I shall go a step farther here and follow the practice 
adopted in treating many of the preceding genera. As in those cases I believe 
this non-committal division of the species into sections will suffice until we are 
in a position to work up the genus monographically. Except in that way it is 
not only difficult but almost impossible to distinguish nearly related genera in a 
fully satisfactory manner. 
