192 THE PALEONTOLOGY OF MINNESOTA. 
5 [Helopora. 
being marked by a succession of narrow A-shaped furrows and ridges. Occasionally, 
and this is true more especially of the young and slender segments, the rounded 
posterior slope is divided by a central furrow into two small ridges, the same as in 
H. spiniformis and H. mucronata. 
Except the transverse section figured on plate LI, the thin sections prepared of 
this species are not satisfactory. As far as they go it appears that the internal 
structure is not materially different from that of H. spiniformis, (see plate III, figs. 5 
and 6) to which species it is closely related. In H. divaricata the zocecial apertures 
are wider, as are also the troughs into which they open, whilst the vertical ridges 
which separate the rows of zocecia are always a more pronounced feature than in 
H. spiniformis. When segments of the two species having the same diameter are 
compared, those of the latter species will be found to have at least one more row of 
zocecia, and to be more nearly cylindrical. With age the angles become entirely 
effaced, the zocecial apertures relatively smaller than shown in my figures, all the 
interspaces nearly on a level, and the striation almost equal throughout. The lower 
extremity of the segments of H. spiniformis also are always more pointed than in 
LH. divaricata. 
Formation and locality.—Rather rare in the lower third of the Trenton shales, at Minneapolis, Min- 
nesota. 
Mus. Reg. No. 5928. 
Hetopora aLTeRNATA Ulrich. 
PLATE III, FIG. 9. 
Helopora alternata Ulrich, 1890. Jour. Cin. Soc, Nat. Hist., vol. xii, p. 192. 
Segments exceedingly slender, slightly curved, about 5.0 mm. long, and scarcely 
0.25 mm. in diameter; lower extremity obtusely pointed, the upper rounded. Zoce- 
cial apertures oval, nearly direct, comparatively large, about 0.13 mm. in length, 
arranged alternately, four in each cycle, twelve cycles in 2.5 mm., and twelve or 
thirteen in a direct line 5mm. long. Interspaces rather thin, generally appearing 
to be simply rounded, but, with the light coming from the side, two narrow furrows, 
passing in a sinuous manner between the apertures, are to be seen. The result is thin 
peristomes united longitudinally by a thin connecting ridge. 
The exceeding delicacy of the segments and the comparatively large size and 
alternate arrangement of the zocecial apertures of this species causes it to be distin- 
guished without difficulty from all known Trenton forms. 
Formation and locality—Rare at the base of the middle third of the Trenton shales, Minneapolis, 
Minnesota. 
