GENERA OF CH^TETID^ AND MONTICULIPORIDyE. 275 



a different texture and much lighter colour. In these cases, 

 therefore, it would appear that the corallites are not only primi- 

 tively distinct, but that in approaching the surface they do not 

 touch each other at all to begin with, or only to a very limited 

 extent, the ultimate union of the corallites being effected by 

 means of a secondary deposit of calcareous matter. In such 

 forms as these, therefore, the corallites in the deeper parts of 

 the corallum are thin-walled, closely contiguous, and more or 

 less polygonal ; whereas they become much thickened and 

 more conspicuously circular or oval in shape as their mouths 

 are approached. The structure of the wall is, in fact, very 

 similar in these cases to what is observable in Stenopoi'u, 

 Lonsd,, except that the thickening of the tubes is uniform, and 

 is not confined to the production of periodic rings. 



Apart from the evidence of microscopic sections, into which 

 I cannot fully enter here, the permanent non-amalgamation of 

 the walls of the corallites in Monticidipoi'a is shown by the fact 

 that fractured surfaces invariably exhibit the exterior of the 

 tubes. This was long ago noticed by Lonsdale, and was set 

 down by him to the fact that the corallum of Moiiticulipora 

 increased by gemmation, whereas that of Chcetetes produced 

 new tubes by a process of fission. In the latter, therefore, 

 rough fractures exhibit the interior of the corallites. Of the 

 correctness of Lonsdale's observations on this point — as obser- 

 vations — I can entertain no doubt ; but I am not clear that the 

 phenomena are really due to the cause which he assigns. That 

 Chcetetes increases fissiparously is certain ; but I am not sure 

 that gemmation is the regular or exclusive mode of growth 

 amongst the MonticuliporcE. I have formerly expressed the 

 opinion (Ann. Nat. Hist, ser. 4, vol. xviii. p. 86) that certain 

 species of Monticidipora exhibited fissiparous growth ; and 

 though further observations have shown me that I relied upon 

 evidence which admitted of misconstruction, and that gem- 

 mation is the common mode of increase in the Monticuliporce, 

 I am still inclined to think that the phenomena manifested by 

 thin sections show that fission of the old tubes occurs at times 



