﻿American Palceozoic Fossils. 



33 



Named in honor of Prof. A. H. Worthen, the accomplished geolo- 

 gist of the State of Illinois. 



Formation and locality: Lower Carboniferous sandy shales, near 

 Somerset, Ky. The exact height of the strata is undetermined, but 

 they belong either to the St. Louis Group or the lower portion of the 

 Kaskaskia beds. 



Rhombopora elegantula, n. sp (PL I., figs. 3, 3a, 36.) 



Zoarium branching at remote intervals, and consisting of cylindrical 

 stems 0.1 inch, or a little less, in diameter. Zooecia rhomboidal 

 or hexagonal, with small ellipitical apertures placed at the bottom of 

 an expanded and distinct vestibule, which is margined by a faintly 

 granular, thin but distinctly elevated, divisional ridge. At intervals 

 a few cells of slightly larger size than the average may be observed, 

 which disarrange their otherwise regular arrangement in both verti- 

 cal and obliquely intersecting lines. In a vertical direction four or 

 five cells occupy the space of 0.1 inch; in an oblique direction, six 

 cells occupy an equal space. 



Tangential sections (PI. I., fig. 3a) show that the line of demar- 

 kation between adjoining cells is usually marked by a single series of 

 very minute spiniform tubuli, that the cells walls are extremely thick, 

 and the cell cavities more or less unequal in size. 



In longitudinal sections (PI. I., fig. 36) the zoarium is conspicu- 

 ously divided into the two peripheral and axial regions. In the lat- 

 ter the tubes are thin-walledj though not excessively so, and proceed 

 towards the surface with a gentle curve. As they reach, on each 

 side, the peripheral regions, they bend abruptly outward. Here their 

 walls become excessively thickened, and it is for this reason that the 

 true form of the " matured " cell is not often shown, as the section 

 usually affords only a view of the walls. The figure referred to above 

 presents two tubes showing their true form. It seems quite certain 

 that an occasional diaphragm crosses the tubes in the axial region, 

 but as they are not sharply defined in my sections, and might not 

 really be such structures, I have not allowed them to appear in the 

 figure. 



The surface appearance of the cells of this species reminds one con- 

 siderably of Anisotrypa symmetrica, described by me in the last part 

 of this memoir. By consulting fig. 5a, on Plate XIII., a fair idea 

 may also be obtained of the superficial characters of the cells of B. 

 elegantula. 



