SUB-GENUS HETEKOTRYPA. 133 



hazardous ; while even so far as external characters go, there 

 are differences in size and in the relative abundance of the 

 small interstitial corallites, which clearly point to the specific 

 distinctness of the latter. For this, therefore, I now propose 

 the name of M. Ulrichii, in honour of Mr E. O. Ulrich, whose 

 published papers have added so materially to our knowledge 

 of some of the more difficult fossils of the Cincinnati Group. 



In general form and aspect, M. Ulrichii most nearly resem- 

 bles the larger examples of ]\I. gracilis, James, having cylin- 

 drical stems, with a smooth surface, and comparatively minute 

 corallites of two sizes. It is, however, distinguished from M. 

 gracilis by the fact that the corallites do not open obliquely 

 upon the surface, by the thinner walls of the calices, and by 

 the greater abundance of the interstitial corallites. 



As regards its internal structure, tangential sections of M. 

 UlricJiii (fig. 22, c, d) show that the normal corallites of the 

 colony are divided into two distinct groups, which are uniformly 

 intermingled with one another, but differ in size and in other 

 characters. The large corallites are mostly oval or sub-circular, 

 mostly from i-goth to i-iooth inch in diameter, and only par- 

 tially and to a very limited extent in actual contact. Occupy- 

 ing all the intervals between the large tubes are numerous 

 smaller interstitial corallites, which vary much in shape and 

 size, but are always more or less angular. These are never so 

 far developed as to isolate the large tubes, and there is never 

 more than one row of them between any given pair of the 

 latter. In addition to the preceding, there are numerous 

 circular, thick-walled, and darkly-outlined hollow tubuli (" spini- 

 form corallites "), which are intercalated at most of the angles 

 of junction of the normal corallites. With the terminations of 

 these upon the surface I am not acquainted, except in a few 

 instances, in which they appear as a series of short spines. 



The axial region of the corallum, as seen in transverse sec- 

 tions, is very largely developed as compared with the circum- 

 ferential portion, and the corallites are here thin-walled and 

 polygonal in shape, while they are also almost, or sometimes 



