THE NATURE OF CAUNOPORA. 121 



But there are no known species of either Aulopora or Syringopora, the colonies of 

 which, in their normal habit and mode of growth, would correspond in any precise 

 way with the aggregate of tubes of a " Caunopora " or " Diapora" as these latter 

 would be seen when divested of the Stromatoporoid in which they are enveloped. 

 I shall enter into this subject at greater length in discussing the special claims of 

 either Aulopora or Syringopora to be regarded as concerned in the production of 

 " Caunoporce." In the meanwhile it is enough to point out, that whether we select 

 Aulopora or Syringopora as the Coral associated with a Stromatoporoid to form 

 " Caunopora " colonies, or whether we allow both to play this part, we are alike 

 compelled to suppose that the Coral, when living under these conditions of life, 

 entirely modifies its normal habits and mode of growth. This seems to me to be 

 the only way of accounting satisfactorily for the peculiarities of the tubes of a 

 " Caunopora " colony, if we suppose these tubes to belong to any known species of 

 Aulopora or Syringopora. 



6. Lastly, if the theory of the commensalism of" Caunopora" be accepted, we 

 must admit not only that several species of Stromatoporoids are liable to form such 

 colonies, but also that at least two or three species of Corals are concerned in the 

 process. For " Caunoporm " differ from one another, not only as to the structure 

 of the " ground-mass " of the fossil, but also as to the size and other characters of 

 the embedded tubes. 



(b) General Arguments in favour of Commensalism. — The above are the principal 

 difficulties which have to be confronted, if we accept the theory of the commensa- 

 lism of " Caunopora ;" and they are so numerous and so weighty as to form, in 

 my opinion, an ample justification for those who have hitherto hesitated to admit 

 the correctness of the theory. On the other hand, the following are the principal 

 arguments, of a merely general nature, which support the theory of commensalism : 



1. The general aspect of the tubes of "Caunopora" and "Diapora" is 

 extremely like that of the tubes of the Auloporoid and Syringoporoid Corals ; some- 

 times resembling Auloporo3 ; at other times making a close approach to the Syringo- 

 porad. 



2. The tubes have definite thickened walls of their own, quite distinct as a rule 

 from the tissue of the investing Stromatoporoid (Plate X, figs. 1, 2, 6, 7, 8, 9). 

 In all cases, the tubes are uniformly and universally covered throughout with a 

 thin layer of the tissue of the Stromatoporoid, so that it is never possible in thin 

 sections to find any portions of the walls of the " Caunopora-tubes " which are not 

 covered externally by the ordinary tissue of the Stromatoporoid. The proper wall 

 of the tubes may be quite thin, and may be merely represented by a dark line ; but 

 usually the wall is further thickened by an extensive deposit of light-coloured 

 sclerenchyma by which the internal cavity of the tube is much contracted (Plate X, 

 fig. 11). In some thin sections I have been unable to make out any proper wall 



