BEYOZOA. 189 



Helopora.] 



in three rows, occupying as many faces of the segment, the fourth side with three 

 longitudinal strise, and no zooecia. Profile of a segment in an obverse or reverse 

 view, wavy on both sides ; in a lateral view only on one side. 



"Zooecial apertures small, oblique, the posterior margin very prominent, arranged 

 alternately in the three rows, with nine in each, in 2.5 mm. A short ridge from the 

 upper depressed edge of each zooecial 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 zooecia. 



" The oblique zooecial 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; Billings, 1866, (part.) Oata. Sil. Foss. Isl. Antic, 



p. 36; Uleich, 1888, Tlie Amer. Geologist, vol. i. No. 4, p. 231, 1890, 

 Jour. Cin. So& Nat. Hist., vol. xii, p. 191, and 1890, Geol. Surv. 111., 

 vol. viii, pp. 401 and 642. 



Zoaria consisting of numerous, subequal, small, cylindrical segments, articulating 

 terminally, poriferous on all sides. Zooecial 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 longitudinal ridges (section b). 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.) and in 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 reparation. 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 suflBce 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. 



