AFFINITIES AND ZOOLOGICAL POSITION. 73 



culi which have been already described (fig, 9, d). There is 

 . thus created an appearance of a central tube in the interior of 

 the wall ; or rather, what I have here described as the wall 

 might possibly be taken to be really one of the smaller inter- 

 stitial tubes divided longitudinally. Apart, however, from the 

 difficulty of conceiving how the canaliculi could be continued 

 across and through the cavity of an interstitial tube, we have 

 in tangential sections, as previously remarked, the conclusive 

 proof that this is not the case, but that we really have to deal 

 with the wall of the tubes. These sections, in fact (fig. 9, a), 

 prove beyond a doubt that the canaliculi are entirely confined 

 to the walls separating contiguous tubes, whether these be the 

 proper zooecia or the cancelli. 



So far as I have seen, no " tabulae," or but an occasional 

 one, seem to be developed in the outer thickened portion of 

 the tubes ; but it is often possible to recognise the delicate 

 radiating spines or " septa," which are so well displayed in 

 tangential sections. Lastly, owing to the unequal thickening 

 of the walls of the tubes, it is not uncommon for the longi- 

 tudinal section of their cavities to assume a beaded appear- 

 ance, though this is not constant, and, when present, varies 

 much in amount. 



Having now fully considered the structural characters of a 

 recent species of Heteropora, it may be as well to summarise 

 the chief points of resemblance and difference between Hetero- 

 pora and Monticulipora, and to state the general conclusions at 

 which I have arrived as regards the possible relationship of 

 these two genera to one another : — 



(i) As to the general form of the skeleton, the two genera 

 are very similar, though this point is of itself wholly without 

 significance, and the latter genus comprises types of very 

 varied shape and mode of growth. If, however, we compare 

 Heteropora with the ordinary dendroid types of Monticulipora, 

 we have in both a skeleton made up of slender fasciculate 

 tubes, which are nearly vertical in the centre of the branches, 

 and then curve outwards, gently or abruptly, to reach the 



