GENERA OF CH.ETETID^ AND MONTICULIPORID^. 293 



alluded to, they are partially separated by the intervention of 

 the smaller corallites, which are always angular in shape, have 

 thin walls, are never so far developed as to completely isolate 

 all the larger tubes, and are always provided with more nu- 

 merous and more closely set tabulae than is the case in the 

 latter. Type of the group, Monticulipoi-a petropolitana. 

 Pander. 



VI. MoNOTRYPA, Nich. — Corallites of two kinds, which are 

 not conspicuously different from one another. The larger 

 tubes aggregated into clusters or " monticules," and very 

 slightly differing in size from the smaller ones. The smaller 

 tubes occupying all the spaces between the monticules. All 

 the corallites, of both kinds, uniformly thin-walled, regularly 

 polygonal, and similarly tabulate, the tabulae being remote and 

 few in number, and not uncommonly disposed at correspond- 

 ing levels in contiguous tubes. Type of the group, Moriticuli- 

 pora iChcstetes) undulata, Nich. 



Sub-genus Heterotrypa, Nich., 1879. 



This section includes many of the most typical and most 

 familiar of the species oi Monticulipora, comprising among them 

 the M. mammulaia, D'Orb., which, as the species first on the 

 list of MonticuliporcB given by D'Orbigny (Prodr. de Paleont., 

 p. 25), has the right to be considered as the type of the whole 

 genus. In addition to M. mammulata, D'Orb., we must place 

 here M. raj7wsa, E. and H. (PI. XIII., figs. 2-2 a), M. rugosa, 

 E. and H., M. frondosa, D'Orb., M. Jamesi, Nich., M. monili- 

 formis, Nich., M. tumida, Phill., M. gracilis, James, and various 

 other more or less certainly established species. In all these 

 forms the corallum is conspicuously dimorphic (sometimes tri- 

 morphic), and consists of two sets of corallites of different sizes. 

 The larger tubes are subpolygonal or sometimes rounded in 

 shape, and are more or less conspicuously thickened towards 

 their mouths, while they usually possess few and remote 

 tabulae, or may be in great part devoid of these structures. 



