664 PALAEONTOLOGY OF ILLINOIS. 



OELOCONUS Ulrich. 



(For generic diagnosis see page 402.) 



This genus, though closely related to Rhombopora and Rhab- 

 domeson, is amply distinguished by its zoarial and zooecial pe- 

 culiarities, The free character of the zoarium, its hollow coni- 

 cal form, and the comparatively short zooecia, are the chief fea- 

 tures of the genus. So far only two species are known to 

 have these characters. In Bactropora Hall, the zoarium is also 

 free, but neither conical nor hollow, the entire axial region of 

 the cylindrical stems being occupied by the tubular primitive 

 portion of the zooecia. 



CCELOCONUS RHOMBICUS Ulrich. 



PI. LXXII. fig. 4-4o. 



Zoarium a hollow elongate-obconical body, 8 to 10 mm. long, 

 and about 1.5 mm. wide across the open top; lined on the 

 inner side with a thin epitheca. Thickness of the zoarial crust 

 about 0.2 mm. Zooecia apertures differently arranged on differ- 

 ent portions of the surface. Near the pointed base they are 

 narrow and ranged between raised longitudinal lines, which give 

 this portion of the zoarium the appearance of being striated. 

 Farther up they gradually become suboval, occupy the bottom 

 of sharply defined rhombiform sloping areas, arranged in rather 

 regular diagonally intersecting series. Toxvard the upper end 

 the areas gradually change from rhombic to regularly elongate 

 hexagonal, with the ends acute. At the same time an arrange- 

 ment of the areas in transverse series becomes evident. The in- 

 terspaces are rather thin and, in the usual state of preservation, 

 (in which the summit alone projects above the infilling matrix) 

 appear very delicate. At the angles of junction the summit is 

 slightly elevated. Measuring longitudinally, about six zooecia 

 apertures occur in 2 mm.; diagonally, five in 1 mm.; trans- 

 versely, five in 1 mm. 



Thin sections show that the zooecial walls are comparatively 

 thin throughout, that the zooecia are short, ranged in longitu- 

 dinal series on the basal plate, and provided with both hemi- 

 septa, the inferior one but little developed. 



