SURFACE. 61 



These are well seen in such types as Stromatopora concentrica, Gold., var. colliculata, 

 Nich. (Plate III, fig. 5), and Actinostroma verrucosum, Goldf., sp. ; but they occur 

 in various types of diverse affinities. Sometimes these monticules are small and 

 pointed, sometimes they are large and blunt, and sometimes they coalesce into 

 ridges. In some cases they do not appear to be perforated at their summits, and 

 they seem to have no special connection with the astrorhizae. In many cases, 

 however, each monticule corresponds with the centre of an astrorhizal system ; and 

 in such cases each is perforated at its summit by one or more comparatively large 

 apertures (Plate III, figs. 4 and 6). These apertures at the summits of the 

 monticules are what have been regarded as " oscula " by those who, like myself at 

 one time, have upheld the reference of the Stromatoporoids to the Sponges. The 

 possession of perforated monticules is a phenomenon which is specially charac- 

 teristic of such Stromatoporoids as have astrorhizae in regularly superposed groups ; 

 each vertical series having a central canal, from which the astrorhizae of successive 

 laminaj spring, and which ultimately opens on the surface (Fig. 7). Prof. Ferd. 

 Roemer, as formerly pointed out, has endeavoured to explain away the existence 

 of any such openings, as being merely formed by the inclusion in the growing 

 Stromatoporoid of the tubes of Spirorbis ; but I have often seen surface-openings 

 produced in this way, and they are entirely different to those now in question. 

 The latter can be shown conclusively, by means of thin sections, to belong to the 

 Stromatoporoid in which they are found, and to be formed in the way I have above 

 described ; this conclusion being the one which we should have been otherwise led 

 to draw from the great regularity with which these monticules and their openings 

 are disposed in many types. There are, in fact, certain species in which the 

 skeleton may be said to be built up of a series of cylinders, each terminating on the 

 surface by a perforated prominence, and being traversed longitudinally by a median 

 canal from which the astrorhizae are given off. It is, however, to be noted that 

 there are, on the other hand, certain types having well developed astrorhizae 

 arranged in more or less regular vertical rows, but not having the surface 

 covered with monticules corresponding with the centres of the astrorhizae. This 

 condition of things occurs, for example, in Stromatopora typica, Rosen. 



In a great many Stromatoporoids it is not possible to recognise with any 

 distinctness any definite superficial apertures which might have served for the 

 emission of zooids. In a large number of cases this is probably only due to the 

 fact that when these openings are filled with the matrix it becomes difficult or 

 impossible, owing to their minute size, to detect their presence at all, except in 

 specimens preserved in quite exceptional perfection. In other cases, as in young 

 examples of Labechia conferta, Lonsd. (Plate III, fig. 10), the apparent absence of 

 surface-perforations seems to be due to a real want of any apertures, the zouids having 

 been given off from the surface-investment of the coenosarc. In weathered examples 



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