SUB-GENUS HETEROTRYPA. 127 



a less extent, by its general form and proportions. In this 

 last-mentioned respect it is superficially not unlike ]\l. UlricJiii, 

 Nich. (the so-called M. Fletcher i of the Cincinnati Group). 



As regards internal structure, tangential sections (fig. 20, 

 A and b) show the ordinary corallites to be of very small size, 

 and to be completely fused by means of their greatly thickened 

 walls, their general shape being oval, rounded, or sub-poly- 

 gonal. A moderate number of small corallites are developed 

 at the angles of junction of these, and minute dark " spiniform 

 corallites" are developed in some number. Transverse sec- 

 tions show that the central axis of the branches is of com- 

 paratively large size, and that this region of the corallum is 

 occupied by thin - walled polygonal tubes (fig. 20, c), the 

 corallites only becoming thickened in their circumferential 

 zone. Longitudinal sections show that tabulse are very 

 sparsely, or not at all, developed in the axis of the corallum, 

 where the tubes are delicate and thin-walled ; while the outer 

 portions both of these (fig. 20, d) and of transverse sections 

 show that the tabulae are fairly well developed throughout 

 the outer thickened portion of their course, being always 

 horizontal and complete, and being much the most numerous 

 in the small corallites. These sections also show (fig. 20, e) 

 that the walls of the corallites in the outer part of their course 

 have the peculiar fibrous structure characteristic of those 

 Moiitiailipora: in which the walls are seemingly fused together, 

 while they are sometimes seen to be traversed by tubuli repre- 

 senting the spiniform corallites. 



The typical AI. gracilis, James, is a small form, its stems 

 being mostly from a line to two or three lines in diameter. 

 At a higher horizon in the Cincinnati Group, however, there 

 occurs abundantly a larger form, the branches of which range 

 from two to four or five lines in diameter. For this latter, Mr 

 U. P. James has provisionally proposed the name of Chcetelcs 

 Mee/ci (ThQ Paleeontologist, No. i, p. i, 1878). I have, how- 

 ever, made a careful microscopic examination of this form by 

 means of thin sections, and am satisfied that it is merely a 



