58 BRITISH STROMATOPOROIDS. 



the axial tube, as generally in Stachyodes (Plate VIII, fig. 10), or they may be 

 curved, or even regularly funnel-shaped. They often have the form last mentioned 

 in the genus Idiostroma, "Winch., and they then show a curious resemblance to the 

 tabula? in the genus Syringopora. As a rule, the axial tube gives off smaller lateral 

 branches, which ascend in the substance of the coenosteum, dividing as they proceed. 

 These are also furnished with tabulae, and appear to be directly connected with 

 the general interspaces of the skeleton. Whether the axial tube and its ascending 

 lateral branches open finally upon the surface is a point on which it is difficult to 

 obtain conclusive evidence ; but there are strong reasons for thinking that they 

 certainly do so. 



Now, it is certain that these tubes, whatever may be their function and nature, 

 are veritable constituents of the organisms in which they are found. Whatever 

 may be the nature of the " tubes " in " Caunopora" it is not admissible to regard 

 the tubes above alluded to as being parasitic structures, or as otherwise foreign to 

 the Stromatoporoid in which they occur. They hold a definite position in relation 

 to the rest of the organism, they communicate with the cavities of the general 

 skeleton by apertures in their walls, they often give off branching lateral canals, 

 and they are invariably present in the genera which they characterise. These 

 considerations render it certain that these tubes are truly portions of the organisms 

 in which they occur. 



With regard, however, to the function of these axial tabulate tubes in Idiostroma, 

 Amphipora, and Stachyodes, there is not at present sufficient evidence to warrant 

 any very definite hypothesis. Perhaps the most probable theory that we can in the 

 meanwhile form as to their nature is that the main axial tube lodged a stolon or 

 axis of the coenosarc, and that the lateral branched tubes in connection with this 

 were occupied by a special series of zooids. There are, also, some considerations 

 which would render it not wholly unlikely that these tubes were connected with the 

 process of reproduction, and lodged the generative zooids. Apart, however, from 

 all theories as to their nature, it may be pointed out that the existence of such 

 tubes as a constituent portion of the coenosteum of certain Stromatoporoids, 

 deprives the hypothesis that the walled tubes of " Caunopora" also belong to 

 the organism, of part of the inherent improbability that would otherwise 

 attach to it. 



(i) The Epitheca. — In a very large number of Stromatoporoids the under surface 

 of the coenosteum is covered by a thin, imperforate, concentrically striated, 

 calcareous membrane (Plate III, figs. 7, 8, and 9), which has all the characters of 

 the " epitheca " of many composite Corals, and to which the same name may be 

 applied. In microscopic structure it appears to be merely composed of granular 

 calcareous matter. Very many of the Stromatoporoids appear to constantly possess 

 an epitheca, which in general arrangement and appearance is precisely similar to the 



