120 BRITISH STROMATOPOROIDS. 



numerous examples of both Aulopora and Syringopora ; and yet I have never found 

 a single example of either " Caunopora " or " Biapora " in it, nor do I know that 

 one has ever been found. 1 Another but not so striking case is that of the Corni- 

 ferous Limestone of North America, in which we find a remarkable profusion of 

 species of Syringopora, and to a less extent of Aulopora, existing with great 

 numbers of Stromatoporoids ; and yet " Caunoporos " and " Diapora, " are exceed- 

 ingly rare. 



3. The converse of this also holds good. That is to say, there are strata in 

 which " Gaunoporce " and " Diaporce" are very abundant, and in which Aulopora 

 and Syringopora may be very rare. This is most marked in the case of the 

 Devonian Limestones of Devonshire, in which " Gaunoporce " are extraordinarily 

 abundant, whereas species of Aulopora or of Syringopora (unless they are supposed 

 to be nearly all " commensals ") are hardly known, and are certainly very rare. A 

 partial explanation of this may doubtless be found in the difficulties which attend 

 the collection of fossils from these strata otherwise than in polished slabs ; but 

 this explanation would not apply to cases like the Devonian Limestones of Gerol- 

 stein, in the Eifel, where " Gaunoporce" and " Biaporce" are very common, whilst 

 Auloporce are not particularly abundant, and Syringoporm, if they occur at all, are 

 extraordinarily rare. 



4. If we accept the theory of the commensalism of" Caunopora" and " Biapora" 

 we must suppose that the production of the fossils so named involves something 

 very much more than mere envelopment. Perhaps all the forms of the Stromato- 

 poroids — save such abnormal types as Amphipora and Beatricea — occur occasionally 

 encrusting or enveloping foreign organisms. We should therefore expect that any 

 type of the Stromatoporoids might sometimes be found in the " Caunopora-state." 

 On the contrary, it is only the Stromatoporoids of one particular group which seem 

 habitually to give rise to " Gaunoporce " and " Biaporce ;" and it is only certain 

 species in this group which appear to do so. Moreover, the forms which do produce 

 such colonies are mostly non-encrusting types, furnished with a basal epitheca. 



5. Moreover, supposing that " Gaunoporce" and " Diaporce" are the result of 

 the associated growth of a Stromatoporoid and a Coral, there are no Palaaozoic 

 Corals which have even a general correspondence as regards their internal structure 

 with the " tubes " of these fossils, except the Auloporoid and Syringoporoid Corals. 



1 As before mentioned, I have recently found a number of specimens of " Caunopora " in the 

 Upper Silurian Limestones of Oesel, these being the only Silurian " Caunopora" that I have ever seen. 

 At one locality (Kattri-pank) these " Gaunoporce" are associated with numerous examples of Syringopora 

 hifurcata, Lonsd. (= S. reticulata, His.) ; and I thought at first that the former might easily prove 

 to be merely colonies of the latter living commensally with Stromatoporoid colonies. A microscopic 

 examination of both, however, has satisfied me that in this particular instance the embedded tubes of 

 the " Caunopora" are certainly not referable to this particular species of Syringopora, as they differ 

 from the latter both in size and iu their internal structure. 



