SUBDIVISIONS. loi 



some, at any rate, of the groups adopted are natural. There 

 are, however, certain forms which it is very difficult to classify, 

 and the position of these must remain more or less uncertain 

 in the meanwhile. I shall subsequently deal with the groups 

 that I have adopted at greater length, but I may subjoin 

 here a brief synopsis of the sub-genera into which I propose 

 to subdivide the genus MonticiUipora, D'Orb., as above 

 restricted : ^— 



I. Heterotrypa, Nich. — Corallites of two kinds, the larger ones sub-poly- 

 gonal, partially separated by the development of numerous smaller circular or 

 irregularly shaped tubes, of which there is no more than a single row between 

 any pair of large tubes. Walls thickened towards the mouths of the tubes, 

 and often apparently amalgamated in this region. Spiniform corallites usually 

 present, but sometimes wanting. Tabulae conspicuously more numerous in 

 the smaller tubes than in the larger ones. Type of the group, Moiitiailipora 

 mammiilata, D'Orb. (which is also the type of the whole genus). 



{a) Walls of the corallites apparently fused, their duplex character being 

 hardly or not at all recognisable in thin tangential sections. {Ex., M. 

 tutnida, Phill., M. fnatnmidafa, D'Orb., AI. Ulrichii, Nich., AI. gracilis, 

 James, M. Andrewsii, Nich., AI. ra/nosa, D'Orb., and its varieties 

 AI. ntgosa, E. and H., and AI. Dalii, E. and H., AI. i/ioniiiformis, 

 Nich., AI. subpulcheUa, Nich., AI. O'Ncalli, James, AI. twdii/osa, Nich., 

 &c.) 



{[>) Walls of the corallites preserving their duplex character throughout. 

 {Ex., M. Jainesi, Nich., AI. implicata, Ulrich, M. Girvaucnsis, Nich., 

 M. Trentonensis, Nich., AI. Dawsoni, Nich., &c.) 



II. DiPLOTRYPA, Nich. — Corallites of two kinds, with thin, structureless, 

 apparently amalgamated walls ; the larger ones conspicuously polygonal, with 

 comparatively few and remote tabulae. The large corallites may be aggregated 

 at special points into conspicuous clusters (" monticules"), but they are at the 

 same time scattered indiscriminately through the entire colony, and except 

 where forming the groups just alluded to, they are partially separated by the 

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



1 There are two other groups — viz., Trematopora, Hall (Dybowski), and Ditto- 

 pom, Dyb., which appear to represent additional sub-generic divisions oi Monticuli- 

 pora. I have not included these in the following synopsis, as I have no direct 

 knowledge of their characters. I shall, however, give a brief account of the pecu- 

 liarities ascribed to them by Dybowski in an appendix at the end of this work. It 

 is also probable that some of the as yet undescribed forms which have been yielded 

 by the prolific Silurian strata of Ohio to the researches of Messrs Nickles, Ulrich, 

 &c., will be so far different from any of the above as to require the formation of new 

 sections for their reception. Lastly, among the species here described there are 

 several which are so far abnormal that it is quite possible that they will ultimately 

 find a resting-place in some other section of the genus to that in which I have 

 temporarily placed them. 



