508 PALAEONTOLOGY OF ILLINOIS. 



wider intervals in S. separata than in the two Upper Helder- 

 berg forms described by Hall. 

 Position and locality: Hamilton group, Thunder Bay, Mich. 



SCALARIPORA APPROXIMATA Ulrich. 

 PL XLIII, fig. 3. 



This species differs from the preceding in being somewhat less 

 robust, the margins slightly serrated and the cell apertures 

 somewhat more approximated. A more striking peculiarity is 

 that the transverse ridges are very close set, there being about 

 twenty in two cm. 



Position and locality: Hamilton group; occurs with the pre- 

 ceding at Thunder Bay, Mich. 



EVACTINOPORA Meek and Worthen, 1865. 



(Proc. Acad. Nat. Sci. Phila. p. 165.) 

 (For generic diagnosis see page 387.) 



This remarkable genus of Bryozoa is known only from the 

 Burlington and Keokuk limestones by three species in the for- 

 mer and one in the latter. The free habit of growth, the radial 

 arrangement of the bifoliate fronds, and the comparatively 

 large proportion of the non-celluliferous areas are the principal 

 peculiarities of the genus. Although very striking, these differ- 

 ences are nevertheless not so important as they may at first 

 appear, since the minute structure and the elemental construc- 

 tion of the zoarium does not depart in any essential point 

 from the plan that prevails in the family. Thus, while in Cys- 

 todictya, Dichotrypa and Coscinium, the zoarium consists of 

 two simple folia grown together back to . back, and in Prismo- 

 pora, Scalaripora and Glyptopora, of three such double folia 

 diverging from a central line or axis, these are from four to 

 eight radially arranged double leaves in Evactinopora. In the 

 minute structure of the folia there is little or nothing to dis- 

 tinguish the genus from any of the other genera of the family. 



