650 PALAEONTOLOGY OF ILLINOIS. 



larger specimens. Apertures long oval, their longer diameter 

 about 0.14 mm., the shorter 0.07 mm., alternating in the ad- 

 joining rows, with six in two mm. longitudinally, and about 

 five in 1 mm. diagonally. 



A variety of this form, or, perhaps, a closely related species, 

 occurring with it, is represented by figs. 4 and 4a on PL XLV. 

 Its zoarium has a stunted appearance, caused by the frequent 

 bifurcation of the branches. The zooecia apertures are shorter, 

 and there are eight in 2 mm. longitudinally. The granules and 

 ridges are less marked, and, especially near the base, many of 

 the zocecia apertures are closed by a thick calcareous deposit. 

 Should these differences prove constant, the form might be 

 called jR. humilis. 



Position and locality: Upper Helderberg, Falls of the Ohio. 



RHOMBOPORA DICHOTOMA Ulrich. 



PI. LXX, Figs. 13-13b. 



Zoarium dendroid, spread in the same plane, arising from an 

 expanded base. Branches strong, about 3 mm. in diameter, 

 dividing dichotomously at intervals of 10 to 15 mm.; angle of 

 bifurcation about 85. Surface with inconspicuous elevations, 

 which sometimes continue around the branch as low annula- 

 tions. Zooecia apertures arranged in regular intersecting spiral 

 series, crossing each other at an angle of about 90. These 

 lines are somewhat interrupted by the surface elevations, upon 

 which the apertures are appreciably larger than elsewhere. 

 Toward the upper ends of the branches the interspaces are ob- 

 tusely ridge-shaped, the apertures circular, 0.12 mm. in diame- 

 ter, and situated at the bottom of a rhornboidal sloping area. 

 The summit of the ridge with a closely arranged series of small 

 nodes, of which the one at the angles of the rhomb is usually 

 the largest. Toward the base the diameter of the apertures is 

 gradually decreased to 0.08 or 0.09 mm., by the thickening of 

 the interspaces. The latter has also become rounded, and the 

 nodes a little larger and less regularly arranged. Measuring 

 diagonally thirteen apertures occur in 4 mm.; longitudinally 

 seven or eight in the same space. 



This species is related to R. lepidodendroides Meek, from the 

 Upper Coal Measures, but differs in its mode of growth, less 



