194 THE GENUS MONTICULJPORA. 



tinct. There are no interstitial tubes, ^ or proper series of 

 small corallites ; but there is a remarkable development of the 

 sino-ular " spiniform corallites " which constitute such a striking 

 feature in many other species of Monticulipora. These curious 

 structures present their usual characters, each exhibiting a 

 central circular clear spot, surrounded by a thick, laminated, 

 and dark-coloured wall. Usually, one of these tubules is 

 developed at each angle of any given large corallite ; and 

 though they can often be traced in longitudinal sections (PI. 

 IV. fig. 3^) running in the substance of the wall, I have not 

 been able to discover how they terminate upwards, as none of 

 my specimens have the superior aspect of the corallum in a 

 sufficiently well-preserved condition. 



Vertical sections of the corallum (PI. IV. figs. 3^, 3/") show 

 that the corallites are throughout approximately perpendic- 

 ular to the basal epitheca, their walls gradually thickening as 

 the upper surface is approached. Tabulse are always present, 

 and are always complete and horizontal ; but their number is 

 comparatively small, and they often show a periodic develop- 

 ment, being placed at the same level in adjoining tubes. 



In the general conformation of the corallum, M. discoidea, 

 James, is most nearly related to M. Newberry i, Nich., and 

 M, calyaila, James, though quite distinct from these in in- 

 ternal structure. Thus, AI. Newberryi, Nich., has the colony 

 divided into two quite distinct sets of corallites, the large 

 tubes having incomplete tabulae, while the small ones have 

 close-set complete tabular. AI. ca/ycitla, James, on the other 

 hand, has a much thinner corallum than Af. discoidea, and 



^ I formerly (Ann. Nat. Hist., ser. 4, vol. xviii. p. 89, PI. V. fig. 7) described and 

 figured small corallites as existing in AI. discoidea, James ; but though my Jigure 

 was correct, my conclusion therefrom was erroneous. I was not, namely, at that 

 time aware of the value and importance o{ fa/igential secUoxxs, and the section that 

 I described and figured was a transverse one, passing through the corallum parallel 

 with its under surface and just above the line of the epitheca. Consequently, it 

 intersected the corallites at a level just above the point where they spring from the 

 basal plate, and where many of them are quite young. Hence the appearance 

 of a series of small corallites intermixed with the larger ones. Hence, also, the 

 absence in the section which I figured of the curious spiniform tubes which are 

 so conspicuous in tangential sections. 



