﻿Atnerican Palceozoic Fossils. 



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one species; in the Lower Helderberg group by two; in the Devonian 

 by two or more; in the Lower Carboniferous formations by at least 

 six; and in the Coal Measures by three. Of these have been described 

 only the two Lower Helderberg species by Hall (as species of Tremato- 

 pora), and the type of the genus by Meek. Following will be 

 found descriptions of five Lower Carboniferous species and one Coal 

 Measure species. 



Rhombopora lepidodbndroidea, Meek. (PI. I., figs. 1, la and 16.) 



Bhomboporalepidodendroides, Meek. Pal. Eastern Nebraska, p. 141, Plate VII., figs. 2, 

 a, 5, «, d, e, f. 



This species forms ramose slender, cylindrical, remotely bifurcating 

 zoaria, the average diameter of which is about .07 inch, the largest 

 fragment seen having a diameter of .12 inch. Distance between bi- 

 furcations varying from .30 to .90 of an inch. Zocecia arranged very 

 regularly in quincunx, forming vertical, transverse and intersecting 

 spiral series around the branch, with broadly elliptic apertures, placed 

 at the bottom of distinctly impressed and expanding vestibules of 

 rhombic form, the margins of which are occupied by a single, occa- 

 sionally a double, series of small spiniform tubuli, and generally at 

 each angle by one of much larger size. In the usual state of preser- 

 vation, the small spines are not apparent. Measuring in the direction 

 of the oblique rows, seven or eight cells occupy the space of 0.1 inch. 



In tangential sections (PI. I., fig. 1) the cells are elliptical, about 

 their own diameter distant from each other, with apparently amalga- 

 mated walls. The line of contact between adjoining cells is marked 

 usually by a single row of minute dark spots, which, under favorable 

 circumstances, enclose another very small lucid spot, proving them to 

 have been hollow, while at the ends of the cells, almost invariably, 

 occurs a much larger thick-walled tubulum. These represent the two 

 sizes of granules or spines observed at the surface. 



Longitudinal sections (PI. I., fig. la) show that in the axial or "im- 

 mature " region the tubes have thin walls, that they rapidly attain their 

 full size, and slope outward almost immediately after being developed. 

 Near the surface the direction changes until it is nearty at a right angle 

 with the plane of the surface. In the peripheral region their walls are 

 abruptly thickened, and composed of thin superimposed layers, which 

 slope dowward on each side to the visceral cavity, and indicate the 

 form of the vestibules at previous stages of growth. Beside a larger 

 number of slender, apparently interrupted, dark lines, representing 

 the small spiniform tubuli, a section will show a limited number of the 



