204 THE GENUS AIONTICULIPORA. 



Large coralHtes more or less markedly prismatic, their size 

 being variable in different specimens, though tolerably uniform 

 in any given example. They vary from i-5oth to about 

 i-8oth inch in diameter, but they seem usually to be about 

 i-6oth to I -50th inch across. The small tubes are exceedingly 

 variable in size and shape, but are always angular. The 

 large tubes are furnished with the peculiar incomplete tabula; 

 which are characteristic of the group, while the small tubes 

 have very closely-set complete and horizontal tabulse. Walls of 

 the tubes thin, structureless, and apparently fused. Corallum 

 free, provided basally with a concentrically wrinkled epitheca. 



Obs. — In general form and aspect (fig. 42), the corallum of 

 Prasopora Grayer closely resembles that of moderately young 



examples of Monticiilipora {Diplotry- 

 pa) petropolitana, Pand., with which 

 a merely external examination would 

 almost certainly lead the observer to 



Fig. 42. ^A, Under surface 'of a . -t-i • i r rr ^ 



small specimen of Prasopora plaCC it. 1 hlS, therefore, attords an- 



Graya:, natural size, showing the ^^^J^^^. example of the USelcSSUeSS of 

 epitheca ; B, Side view of an- ^ 



other, larger specimen, of tiie attempting to decide the true structure 



natural size. i . . ^ ■» ^ • i • • i 



and position oi any Monticuliporoid 

 by an appeal to its form and general appearance. The coral- 

 lum is conspicuously and very remarkably dimorphic, the large 

 and small corallites being uniformly distributed throughout the 

 entire colony, and being singularly different in internal struc- 

 ture. The structure of the large corallites is most easily recog- 

 nised in longitudinal sections (fig. 43, b), in which each is seen 

 to possess a central tube, occupying the axis of the visceral 

 chamber, and entirely surrounded by a circumferential zone of 

 peculiarly modified tabulse. The central tube may be open 

 throughout, but it is usually intersected, at remote intervals, 

 by delicate horizontal tabulae. Surrounding the central tube 

 on all sides, and forming its walls, is a zone of tabulae, which 

 spring from the wall of the corallite, and are then bent down- 

 wards so as to become parallel to the long axis of the corallite, 

 finally joining the next tabula; below. There is thus formed a 



