SUBGENUS IIETEROTRYPA. 125 



Monticulipora (Heterotrypa) gracilis, James. 

 (Fig. 20, and PI. 11. figs. .1, i^.) 



Chatties gracilis, James, Cat. Foss. Cincinnati Group, 1871 (named, but 



not figured or described). 

 „ „ Nicholson, Quart. Journ. Geol. Soc, vol. xxx. p. 504, PI. 



XXIX. figs. 7, 7^, 1874. 

 „ „ Nicholson, Pal. Ohio, vol. ii. p. 198, PI. XXI. figs. 8, 



8^, 1875. 

 „ „ Nicholson, Ann. and Mag. Nat. Hist., ser. 4, vol. xxviii. 



p. 90, PI. V. fig. 13, 1876. 



Spec. Char. — Corallum dendroid, the branches cyhndrical or 

 sub-cylindrical, from less than a line to three lines or more in 

 diameter, dividing dichotomously at short intervals. Surface 

 smooth, without monticules, covered by the calices, which are ap- 

 proximately oval, with their long axes corresponding with that 

 of the branch, and which are surrounded by much thickened 

 walls. The ordinary calices have an average long diameter 

 (as regards the aperture) of about i-i45th inch, and they open 

 obliquely to the surface, a moderate number of much smaller 

 interstitial apertures existing between them. 



The corallites in the axis of the branches are vertical in 

 direction, and are thin-walled and polygonal in outline, very 

 few or no tabulae being developed in this region. As they 

 bend outwards to reach the surface, their walls become greatly 

 thickened, and tabuke are now abundantly developed, these 

 structures being always complete and horizontal. The coral- 

 lites, in the outer portion of their course, are seemingly com- 

 pletely amalgamated by their walls, and three distinct types of 

 tubes may be recognised amongst them. Firstly, there are 

 the normal tubes, which have an average diameter (as regards 

 their visceral chambers) of about i-i45th inch or less, any 

 given pair of tubes occupying a space of about i-6oth or 

 I -50th inch. These corallites have a moderate number of 

 horizontal tabulae, which in some cases are much reduced in 

 amount. .Secondly, intercalated with the preceding, we find 



