654 PALEONTOLOGY OF ILLINOIS. 



diaphragm, that the superior herniseptum is strong, and that 

 delicate diaphragms are often preserved in the vestibnlar region 

 of old examples. In tangential sections the appearance at differ- 

 ent stages in the development of the zoarium, as above described, 

 are easily traced out. 



This species is distinguished from R. dichotomy and its Keokuk 

 representative, by its smaller zooec-ia and the absence of the 

 strong acanthopores between the ends of the area. This feature 

 is particularly prominent in the Keokuk examples which I have 

 provisionally arranged with the Burlington species. 



Position and locality: Keokuk group; near Plymouth; near 

 Whitehall, in Greene Co.; Warsaw and Nauvoo; all localities in 

 Illinois. 



RHOMBOPORA TRANSVERSALIS Ulrich. 



PI. LXXI, fig. 4-4b. 



Zoarium a comparatively robust stem, 2.5 to 4 mm. in diam- 

 eter, branching dichotomously, at intervals of 12 mm. or more. 

 Angle of bifurcation generally about 60. Young branches com- 

 monly found flattened from pressure. The surface of the branches 

 is marked by transverse annulations, about 1.7 mm. apart. 

 They are formed by the periodic channel-like prolongation of the 

 anterior end of the depressed areas of transverse series of zocecia 

 apertures. In old examples, in which the interspaces are thicker 

 and more rounded than in young specimens, the annulations are 

 the most distinct, appearing like narrow and slightly depressed 

 solid bands. In young examples they are much obscured by the 

 ridge-like prominence of the interspaces, but as their position 

 is always indicated by a slight elevation of the surface, the 

 feature constitutes a reliable distinctive character. Interspaces 

 thick and rounded in the mature specimens, less so, even rather 

 thin and ridge-shaped in those that are younger; and carrying 

 from one to three series of small closely crowded granules. De- 

 pressed areas, (L e., the space bounded by the summits of the 

 interspaces,) more or less lozenge-shaped, this shape prevailing 

 especially in young examples. At the bottom of the areas are 

 situated the ovate apertures, whose long diameter is about 0.12 



