88 THE GENUS MONTICULIPORA. 



larger or smaller number of well-developed complete tabular 

 (fig. 14, B and e). On the other hand, the interstitial tubes 

 are not provided with any tabulce, instead of being closely 

 tabulate, as they would be in any species of Hetcrotrypa or 

 FisttUipora among the Monticuliporoids ; and they appear, 

 indeed, to be quite superficial, and not to extend inwards to 

 such an extent as to render them a conspicuous feature in 

 sections which cut the tubes longitudinally. Moreover, there 

 are grounds for believing that distinct apertures are present 

 in the walls of the species of Ccramopora in question, though 

 I do not feel sure that this is the case. The above differences 

 in structure, however, combined with the different external 

 aspect, are sufficient to enable us to effect in general a satis- 

 factory separation between the species of Montictdipora and 

 any of the known forms of Ccramopora, Hall. 



With regard to the Polyzoan genus Heterodictya, Nich., 

 the only necessity for entering into any comparison of its 

 structure with that of Alonticidipora arises out of the fact that 

 such an excellent observer as Mr E. O. Ulrich has referred 

 (Cat. Foss. Cincinnati Group, 1880) the form which I believe 

 to be Alonticiilipora pavonia, D'Orb., to the former. I am, 

 however, quite unable to accept this conclusion, though I do 

 not feel disposed to assert positively that M. pavonia, D'Orb., 

 with its various abnormal characters, is an indisputable Mon- 

 ticuliporoid. In Heterodictya, Nich., we have to deal with an 

 undoubted Polyzoon, of the general type of Ptilodictya. The 

 colony forms a thin falciform expansion which is composed 

 of two layers of tubes diverging in opposite directions from 

 a definite striated median calcareous lamella, and which is 

 bounded by perfectly definite margins, and has a perfectly 

 definite shape. As regards internal structure, the only re- 

 semblance between Heterodictya and any of the Monticuli- 

 poroids is to be found in the existence in the former of well- 

 developed tabulae (fig. 15, a and b), which, however, are often 

 incomplete. No interstitial tubes are present, nor are there 

 any "monticules" or structures comparable with these. 



