GENERAL AND COMPARATIVE STRUCTURE. 49 



ally disposed lamellae of dense sclerenchyma. The central 

 cavities of the spines seem, however, to be more or less com- 

 pletely obliterated with age ; and the corallites in the outer 

 portion of their course (fig. 6) exhibit the annular thickenings 

 of their walls which are so characteristic of the genus Stenopora. 

 In spite of these differences, the resemblance between the 

 spines of the Monticuliporcs above alluded to and the similar 

 structures in certain species of Stenopora is so striking that one 

 can hardly resist the conviction that there must subsist be- 

 tween the two a relationship of real affinity. 



V. Dimoiphisin of the Coralhim. — Apart, however, from 

 the very curious structures that I have just spoken of under 

 the name of *' spiniform corallites," microscopic examination 

 brings out very clearly the important fact that the corallum in 

 Monticidipora is in general dimorphic, and consists of two 

 different sets of corallites, which must, during life, have been 

 inhabited by different sets ofzooids. The existence of minute 

 tubes, either scattered among the larger ones, or aggregated into 

 special groups, has, of course, been long known to palaeontolo- 

 gists ; but these have, for the most part, been regarded either 

 as merely young corallites or as " coenenchymal tubuli." ^ Simi- 

 larly, palaeontologists have long known that certain species of 

 Monticidipora {e.g., M. pnlchelia, E. and H.) exhibit groups 

 of large tubes distributed at intervals among those of average 

 size ; but the true import of these appearances hardly admitted 

 of recognition save by the light of Mr Moseley's researches 

 upon the living Hcliopora. I have, however, now thoroughly 

 satisfied myself that the corallum in Monticidipora is, in gen- 



1 Until the publication of Mr Moseley's researches upon the structure of the re- 

 cent Heliopora (Phil. Trans., vol. clxvi. p. 92, 1876), the closely tabulate interstitial 

 tubes of Heliolites and allied forms were generally, and very naturally, regarded 

 as being of the nature of a vesicular " coenenchyma." If, however, the inter- 

 stitial tubes o{ Heliolites are to be regarded as formed by a special series of modi- 

 fied polypes — as Mr Moseley's researches seem to render certain — then we cannot 

 avoid the conclusion that the interstitial tubes of so many of the Monticuliporoids 

 are essentially of the same nature. We m.ust therefore abandon the term of 

 " coenenchymal tubules," as applied to the small interstitial corallites of Mon- 

 ticiclipora and its allies Fistiilipora, M'Coy, Constellaria, Dana, &c. 



D 



