248 TABULATE CORALS. 



larger corallites were prolonged exteriorly into " costse " (Pol. 

 Foss. des Terr. Pal., p. 223); but I do not think that any 

 structures to which this name could be properly applied are 

 really present. On the contrary, I believe that the apparent 

 " costse " are in reality as also in Plasmopora nothing more 

 than the imperfectly-developed walls of the smaller interstitial 

 corallites. In the structure of the large corallites, Propora 

 differs in no respect from either Heliolites or Plasmopora; 

 and as regards the smaller corallites, the condition of parts 

 is very similar to that which we have seen to exist in the 

 latter genus. I have not had the opportunity of examining 

 very young specimens of Propora tubulata-, but if we look 

 at a thin longitudinal section of an adult example (PI. XI., 

 % 3 ^) we see that the interspaces between the larger coral- 

 lites are occupied by a vesicular tissue, composed of lenticular 

 vesicles of different sizes, closely resembling the vesicular tissue 

 of a Cystiphyllum in general aspect. This tissue resembles that 

 of Plasmopora, except that the vesicles are more regular in form ; 

 and I entertain no doubt but that it is similarly formed by the 

 lateral anastomosis and confluence of the convex tabulae of the 

 interstitial tubes or small corallites, with the resulting oblitera- 

 tion of their walls. The walls of the small corallites, in fact, 

 can occasionally be very partially detected, though more usually 

 they have entirely disappeared as distinct structures. Similarly, 

 when we come to examine thin tangential sections of Propora 

 (PI. XI., figs. 3 and 3 a), we see that the narrow interspaces 

 between the comparatively close-set large corallites are occu- 

 pied by rows of irregular spaces, bounded by dark lines. 

 These lines, however, are not " costae," but are the cut edges 

 of the lenticular vesicles formed by the coalescence of the 

 curved tabulae with the walls of the small corallites. Ordin- 

 arily there is but a single row of small corallites between each 

 pair of large ones, though sometimes two rows are present. 

 (In long sections, as in PI. XI., fig. 3 b, there commonly 

 appears to be quite a wide tract of vesicular tissue between 

 two contiguous large tubes ; but this, of course, is only due 



