i62 THE GENUS MONTICULIPORA. 



were taken from the same specimen ; and, as I know now, 

 I unfortunately took my horizontal sections from the true 

 J\T. Whitcavesii, and my vertical ones from a similar-looking 

 but really different specimen (yl/. Selzvynii, mihi). Nor did 

 the error stop here ; for I had at the same time prepared 

 vertical sections of another set of specimens, from the Cin- 

 cinnati Group of Ohio, which I believed to be the same as 

 the preceding-, and which are really identical with one of the 

 Trenton Limestone types (J/. Sc hey nil), or at most a well- 

 marked variety of it. One of the vertical sections of the 

 Cincinnati Group specimens, by a singular mischance, became 

 mixed with the other sections, and was accordingly wrongly 

 labelled as M. Whitcavesii, and duly figured as such (Pal. Tab. 

 Cor., PI. XIV. fig. i). Having recently had occasion to make 

 an entirely fresh series of sections from the specimens in 

 question, I was enabled to unravel this complicated series 

 of mistakes ; and I am therefore now able not only to give 

 a correct statement of the true characters of M. Whiteavcsii, 

 and to define its sinmlacrttni (viz., M. Sclzfynii), but also to 

 give the solution of one or two difficulties which, in the former 

 confusion, had appeared to me to be absolutely inexplicable. 



Average examples of the true M. Whitcavesii, Nich., are 

 in o-eneral discoidal, sometimes considerably elevated, their 

 diameter varying from less than half an inch up to an inch 

 and a quarter or more, and their height from two to six lines. 

 The base of the corallum may be simply flat, but is in general 

 deeply concave (fig. 31, b), and is always covered with a well- 

 marked striated epitheca. Thin sections, taken in any direc- 

 tion, show that the corallites are uniformly thin-walled, not 

 being thickened as they approach the surface, and being so 

 amalgamated with each other that the originally duplex char- 

 acter of the wall cannot be recognised at all. 



Tangential sections vary somewhat in the appearances which 

 they present, according to the precise depth below the surface 

 at which they intersect the corallum. When taken just below 

 the surface (fig. 31, c and d), the corallum is most clearly seen 



