liO ME. H. M. BEE5AED ON RECENT PORITIDJE. 



Let US then see what are the arguments in favour of this 

 suggestion. Per the moment, dealing only with 'Pontes, we find 

 the polyps, like their calicles, small and degenerate, i. e. little 

 grown and with only twelve tentacles. Their skeletal secretions 

 are purely basal, and the animals retreat down upon them rather 

 than into them*. Thus, in relation to the animal, the skeleton 

 is but feebly developed, so feebly indeed that the coralla rarely 

 have any elasticity or beauty of form. They are, for the most 

 part, merely rounded massive concretions such as could be built 

 up of small flat discs. 



It may perhaps be objected that such a poorly developed and 

 rudimentary skeleton might also be primitive, but this is certainly 

 not the case here; for a glance at fig. 1 shows that the skeleton 

 of Poriies belongs to the highest known type, viz., that in which 

 an internal theca has replaced the primitive epithecal cup. This, 

 as above stated, 1 believe it could only have done by the internal 

 theca-being pronounced enough to replace the epithecal cup as 

 a more efficient refuge into which the polyp could contract. 

 Hence we can only account for the internal theca of Porites by 

 assumiug that it was at one time tall and deep, forming with its 

 jagged septal edges a stronger and better guarded receptacle for 

 the polyp than the primitive epitheca. In other words, the 

 theca of Porites must at one time have been tall and composed 

 of lamellate septa, and the fossa, now shallon- and quite incapable 

 of containing the polyp, must at one time have been large enough 

 to have allowed the whole polyp to sink down into its recesses 

 (fossa, interseptal and intercostal spaces). The theca, from being 

 a true calicle, has become, in Porites, a mere basal pedestal for 

 the comparatively speaking tall polyp which secretes it, it being 

 one of the peculiarities of Porites (and of Goniopora) for the 

 polyps to rise high above their skeletons. 



The internal theca of Porites can therefore only be regarded 

 as rudimentary. It is not a vanishing structure, but it belongs to 

 the most specialized type of Madreporarian skeleton, secondarily 

 arrested in its development. This interpretation is further 

 confirmed by comparing the skeleton of Porites with that of 

 almost any minute young single coral, such as is frequently found 



* Thurston describes the polyps of Porites wliich can no longer retreat into 

 their calicles as protecting themselves when exposed by a layer of slime. 

 Bulletin of Madras Government Museum, No. 3 (1895), p. 93. 



