660 PALAEONTOLOGY OF ILLINOIS. 



single row of small, closely set nodes, some that are of larger 

 size, and usually situated between the ends of the suboval areas. 



Vertical sections are similar to those of R. tabulate, but the 

 diaphragms seem comparatively more numerous. The zooecial 

 tubes seem also to be shorter. Externally R. minor differs from 

 that species in its smaller zooecia and row of closely set nodes 

 on the interspaces. The zoarium is also much smaller. R. per- 

 similis has ridge-shaped interspaces, much more regularly ar- 

 ranged zooecia, and, so far as known, no diaphragms in the 

 zooecial tubes. 



Position and locality: Chester group. Sloan's Valley, and 

 Litchfield, Ky. 



RHOMBOPORA TENUIRAMA Ulrich. 



PL LXX. flg. 8-8b. 



Zoarium very small, consisting of exceedingly slender stems, 

 which may divide dichotomously or send off branches at right 

 angles. Diameter of stems varying between 0.4 and 0.5 mm. 

 Zooecia apertures oval, about 0.11 mm. long, from one-half to 

 two-thirds as wide, arranged in longitudinal and diagonally in- 

 tersecting series, between somewhat flexuous, subangular, longi- 

 tudinal ridges, which have a more or less irregular appearance 

 because of the unequal distribution of the two sizes of spines 

 carried by them. Sometimes the smaller spines are in contact, 

 at other times widely separated. The areas are more or less 

 confluent, with the sides rather wide and slightly convex. The 

 spiral rows of apertures intersect at an angle of about 75, 

 and it requires nine or ten of the apertures to make one volu- 

 tion about the stem. Measuring longitudinally, eleven aper- 

 tures occur in 4 mm. 



The exceeding minuteness of the zoarium, the comparatively 

 strong spines, and the longitudinal arrangement of the zooecia 

 between strong ridges, are the distinctive features of this spe- 

 cies. R. minor is larger, and has the zooecia differently arranged. 

 R. angustata, of the Keokuk group, has much larger zooecia, 

 and does not branch at such short intervale. 



Position and locality: Chester group. Kaskaskia, Illinois, 

 and Sloan's Valley, Ky. 



