192 THE PALEONTOLOGY OF MINNESOTA. 



LHelopora. 



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 III, 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. dimricata the zooecial apertures 

 are wider, as are also the troughs' into which they open, whilst the vertical ridges 

 which separate the rows of zooecia 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 

 zooecia, and to be more nearly cylindrical. With age the angles become entirely 

 effaced, the zooecial 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 

 H. divaricata. 



Formation and locality.— &sX\i^i rare in the lower third of the Trenton shales, at Minneapolis, Min- 

 nesota. 



Mus. Beg. No. 5928. 



Helopoka alternata TJlrich. 



f 



PLATE III, FI&. 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. Zooe- 

 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 5 mm. 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 zooecial 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. 



