BRYOZOA. 437 



Anisotrypa on the other, we see the closest possible relations. 

 Going a little farther we arrive by easy gradations into most 

 close proximity to such undoubted Bryozoa as Rhombopom. 

 Indeed in practice it is not an easy task by any means to draw 

 the line between these genera, though the typical forms appear 

 to differ widely. It will require nothing less than a rnonograph- 

 ical study to determine the exact limits of each division. 



STENOPORA AMERICANA Ulrich. 



PI. LXXIV, fig. 1-la. 



Zoarium ramose, branches large, subcylindrical, irregularly 

 divided, twenty-five to forty mm. in diameter, and twelve cm. 

 or more in height. Usually they are very much flattened by 

 pressure, and appear to be frondescent. Surface sometimes 

 smooth, generally marked by somewhat elevated, broad tuber- 

 cles, whose summits are about four mm. apart. Zooecia with 

 rounded or polygonal apertures varying according as the in- 

 terspaces are thick or thin; those on the monticules a half 

 larger than the others; nine or ten of the ordinary size in three 

 mm. Zooecial tubes thin-walled and vertical in the axial region, 

 then bending rapidly outward proceed direct to the surface; as 

 they enter the peripheral region their walls gradually exhibit the 

 moniliform thickenings characteristic of the genus. These soon 

 become more pronounced and the intervals shorter. Near the 

 surface five to eight occur in two mm. Diaphragms horizontal, 

 perforated. Mesopores absent or very few. Acanthopores of 

 moderate size, fairly abundant, giving to the well preserved sur- 

 face a spinose aspect. 



Position and locality: Keokuk group; Warsaw, and Jersey 

 Co., 111. Not uncommon. 



var. VARSOVIENSIS Ulrich. 



PI. LXXIV, fig. 3, 3a. 



This variety has smaller zooecia and much thicker walls than 

 the typical form. The moniliform thickenings of the walls are 

 also less distinct from each other. The specimens are from 

 Warsaw, 111. 



