126 BRITISH STROMATOPOROIDS. 



than those usual in Aulopora, would nevertheless answer sufficiently well to the 

 tabulas seen in species of the latter genus. Many Auloporce, in fact, have a mixture 

 of curved or straight tabulas with vesicular or funnel-shaped tabulae, such as occur 

 so commonly in " Caunoptora-tuhes." 



Moreover, when we meet, in a single locality, with examples of " Gaunopora " 

 and " Diapora," which differ from one another in the sizes of the embedded tubes, 

 irrespective of the nature of the " ground-mass," then it is not unusual to find 

 free colonies of different species of Aulopora, differing from one another in having 

 differently-sized tubes, in the same locality. 



One great argument, however, against accepting Aulopora as the commensal 

 of "Gaunopora" and " Diapora" is that though Auloporce with differently-sized 

 tubes occur in strata where the latter fossils also have tubes of different sizes, 

 there are no known species of Aulopora in the Devonian Rocks which have tubes 

 so small as those of certain Caunoporce (viz. about ^ mm. in diameter). In the 

 case of such types, therefore, we have the same difficulty in taking Aulopora as the 

 commensal of " Gaunopora " that I have shown to exist in the case of Syringopora. 

 We should have, namely, to suppose that certain of the Auloporce concerned in 

 the production of " Caunoporce" and "Diaporce" are types not known to exist in 

 the free condition. 



The corallites of Aulopora are also not known to possess any septal spines, 

 whereas certain " Caunopora-tubes " undoubtedly possess these structures. Again, 

 free colonies of Aulopora (i. e. colonies merely attached by their lower surface) 

 do not send up straight vertical tubes such as are seen in " Gaunopora " and 

 "Diapora;" nor do the tubes, once produced, become connected by horizontal 

 tubes or cross-branches. Lastly, colonies of Aulopora are very abundant in both 

 Silurian and Devonian strata, growing on the upper or under surface of Stromato- 

 poroids, but not giving rise to " Caunoporce " or " Diaporce." 



The difficulties which attend the hypothesis that the " tubes " of even the 

 laminar forms of " Gaunopora " and " Diapora " are referable to Aulopora, are well 

 exemplified by such a type as Stromatoporella (Diapora) laminata, Barg., which occurs 

 in great numbers and in wonderful preservation (showing both its upper and lower 

 surfaces in perfection) in the quarry of Biichel, in the Devonian Limestones of the 

 Paffrath district. This interesting type forms laminar expansions, often of great 

 size, and completely covered below with a striated epitheca, being only very rarely 

 incrusting. The ccenosteum varies in thickness from 2 — 3 mm. up to 2 — 3 cm., 

 according to the age of the colony. Whatever the thickness may be, the under 

 surface shows no signs of the tubes, whereas the upper surface shows the circular 

 apertures of the tubes distributed uniformly and at tolerably regular intervals, and 

 having their margins just level with the last-formed layer of the Stromatoporoid 

 (Plate X, fig. 3). Vertical sections further would show that, whether the 



