110 BRITISH STROMATOPOROIDS. 



Diphyphyllum stramincum, Bill. Very commonly a portion of a single stem will 

 be covered in this way by a smooth calcareous envelope, while other portions, 

 from natural or artificial deficiency of the membrane in question will exhibit the 

 apertures of the zooidal tubes. In transverse sections of such specimens as possess 

 this membranous covering we find that it is not applied directly to the poriferous 

 surface below, but that between the two are developed numerous large-sized 

 lenticular vesicles, the general appearance of which is not unlike the vesicles of 

 such Corals as the Gystiphylla (Plate IX, fig. 3). To begin with, I was under the 

 impression that these " marginal vesicles " were structures of constant occurrence 

 and that their non-existence in certain specimens was only due to the fact that 

 the peripheral vesicles had been decorticated prior to f ossilisation. I am, however, 

 now satisfied that this is not the case, but that there exist under Amphipora ramosa two 

 distinct groups of specimens, those of the one group, seemingly the most numerous, 

 exhibiting a poriferous and vermiculate surface ; while those of the other group 

 have their original surface surrounded by a zone of vesicles which are in turn 

 enveloped by a thin calcareous pellicle. The only conjecture which I can offer as 

 to the nature of these " marginal vesicles " is that they are reproductive in function, 

 and that they correspond with the " ampulla? " of the Stylasteridce. This view 

 would not only explain the fact that these vesicles were not universal in their 

 occurrence in A. ramosa, but would also throw some light upon the otherwise 

 inexplicable phenomenon that various Stromatoporoids have so commonly portions 

 of the surface covered by a kind of calcareous pellicle. 



VI. THE NATURE OF " CAUNOPORA." 



The singular fossils for which the generic names of " Oawnopora" Phill., and 

 " Diapora," Barg., have been proposed are known, to their cost, by all students of the 

 Stromatoporoids. They have proved a fertile source of differences of opinion ; 

 and these differences are important, since the conclusions which are to be formed as 

 to the structure and relations of the whole group of the Stromatoporoids necessarily 

 depend largely upon the views which may be held as to the nature of the so-called 

 " Caunoporce " and " Diajwrce." As is well known, the fossils to which these names 

 have been given, resemble in all essential respects the ordinary Stromatoporoids, 

 except that the ccenosteum is traversed by numerous thick-walled tubes, which are 

 directed at right angles to the concentric laminae of the fossil, and which open by 

 definite rounded apertures upon its surface. Sometimes these tubes — which may 

 in the meanwhile be conveniently called " Caunopora-tubes " — have simply a thin, 



