THE NATURE OF CAUNOFORA. 127 



ccenosteum be thin or thick, the tubes arise from a level a little above the 

 epitheca, and are continued in an essentially vertical course through the whole 

 thickness of the Stromatoporoid to terminate above in the rounded apertures on 

 the surface. Now, it is quite clear that in this case the epitheca and the first 

 layer of the Stromatoporoid must have existed before the " Diapora " tubes were 

 produced. On the theory of commensalism, therefore, we must imagine that the 

 Stromatoporoid after forming its original epitheca, and one or more of its first 

 laminae, became covered by an Aulopora-colcmy. This latter must have covered 

 the greater part, at any rate, of the upper surface of the Stromatoporoid, and must 

 have produced its first set of tubes with great regularity. Then, as the Stromato- 

 poroid continued its growth by the upward extension of its pillars and by the 

 formation of fresh lamina?, the Aulopora must have lengthened its tubes to a 

 corresponding extent, the tubes growing up in a vertical direction, and always 

 keeping pace with the Stromatoporoid, in such a way that the mouths of the tubes 

 were always just flush with the last-formed layer of the Stromatoporoid. More- 

 over, every now and then horizontal stolons would be thrown out from the lips 

 of the tubes and would become connected with the lips of neighbouring tubes. 

 If, therefore, we removed the enveloping Stromatoporoid, and could examine the 

 embedded tubes alone, we should find a creeping and very regularly-developed 

 network of horizontal tubes, which at tolerably regular intervals would throw up 

 straight vertical tubes, which would be tolerably equal in length and would be 

 joined at different levels by a variable number of horizontal connecting-tubes. 

 The appearances just described differ, however, to a serious extent, from anything 

 that we know of in any species of Aulopora when having its normal mode of growth, 

 and when it is attached parasitically to the exterior of any foreign organism such 

 as a Stromatoporoid or a Coral. Under ordinary conditions, namely, an Aulopora- 

 colony has a very irregular mode of growth, generally forming loose straggling 

 networks, which throw up tubes at irregular intervals. Furthermore, the calices 

 in such a colony are reclined; they do not show any tendency to grow up 

 vertically ; and though they may throw out creeping stolons which in turn become 

 calices, they do not become united with one another by a system of horizontal 

 connecting-processes. 



The above-mentioned differences between an ordinary Aulopora-colonj and the 

 aggregate of tubes of a " Caunopora " or " Diapora " are so striking that we cannot 

 apparently accept of Aulopora as being the Coral which gives rise to these latter 

 fossils, except upon the hypothesis that when living as a commensal with certain 

 types of Stromatoporoids, the Aulopora is forced to completely alter its normal 

 mode of growth. The change in its environment caused by the commensalism 

 must be supposed to induce the Aulopora to enter upon a more active and vigorous 

 as well as a much modified mode of increase. It must be supposed to throw out 



