136 THE GENUS MONTICULIFORA. 



polygonal or sub-polygonal in form, and interspersed among 

 them we see a variable number of much smaller tubes, together 

 with an occasional thick-walled "spiniform corallite." The 

 clusters of small tubes, which are so conspicuous at the surface, 

 are readily recognised in tangential sections, provided these are 

 not made at too great a depth. 



In vertical sections taken at right angles to the compressed 

 surfaces of the corallum (fig. 23, c and d) the corallites are 

 seen to be at first thin-walled, and to radiate outwards on both 

 sides of the middle line, becoming thickened as the surface is 

 approached. Even in the axis of the corallum a moderate 

 number of complete and horizontal tabulce can be detected ; 

 but these become much more numerous in the outer deflected 

 portion of the tubes. It is also to be noted that there is 

 a distinct structural difference between the large and small 

 corallites, the latter having the tabulae much more closely set 

 than is the case in the former. 



From M. Andrczvsii, Nich., the present species is distin- 

 guished by its general form, and by various structural characters 

 of importance, the most striking of these being that the " mac- 

 ulae " are composed of the small interstitial tubes of the colony ; 

 whereas in the former the corresponding parts of the corallum 

 are formed by tubes which are larger in size than the average 

 large corallites. The only species, with which I am acquainted, 

 in which there exist similar clusters of small tubes, is M. 

 Selwynii, Nich., from the Trenton Limestone of Canada. This 

 latter, however, is not only a discoid species, but is separated, 

 among other features, by the possession of incomplete tabulae. 



Horizon and Locality. — Rare in the Cincinnati Group, Cin- 

 cinnati, Ohio. (The specimens in my collection were kindly 

 presented to me by Mr U. P. James of Cincinnati.) 



