36 BRITISH STROMATOPOROIDS. 



and Glathrodictyon) seems to be composed of exceedingly minute granules of car- 

 bonate of lime. In thin sections of such types (Plate I, fig. 1) the skeleton-fibre 

 appears generally to be of a much darker colour than the matrix, and often presents 

 a tolerably uniform cloudy or granular aspect, mostly darkest in the centre, and 

 shading off to a blurred and ill-defined margin. Under high magnifying powers, 

 and in sufficiently thin sections, innumerable minute irregular dark specks, some- 

 times with a clear centre, may be seen to be disseminated through the fibre. The 

 form of these specks is very irregular, and it is their presence which gives to the 

 fibre its cloudy aspect when examined under low magnifying powers. I am in- 

 clined to think that these specks are certainly of the nature of minute vacuities in 

 the fibre, more or less completely filled up with opaque matter, and that they 

 represent the system of minute pores or tubuli which characterise the skeleton- 

 fibre of certain other types. These minute specks are exceedingly well shown in 

 very thin sections of Labechia conferta, in which I shall be able to show that the 

 radial pillars have an unquestionable cribriform structure. 



In no case has any observer succeeded in detecting anything of the nature of 

 definite spicules in the skeleton-fibre of the Stromatoporoids ; and this has always 

 been one of the strongest arguments against the reference of these organisms to the 

 Sponges. 



There are, however, many Stromatoporoids in which the skeletal tissue has an 

 obviously complex character, the nature of which can not be always fully deter- 

 mined. Thus, in all the species of the genus Stromatopora, Goldf., thin sections, 

 taken either tangentially or vertically, exhibit a characteristic dotted or porous 

 structure, the skeleton-fibre being marked with innumerable oval or rounded, clear 

 spaces, surrounded by dark granular tissue (Plate I, figs. 6 and 7). In some cases, 

 as in 8. Carleri, n. sp., S. Beuthii, Barg. (Plate V, figs. 12 and 13), 8. Hupschii, 

 Barg. (Fig. 6), and others, this vesicular structure of the fibre is upon such a large 

 scale as to be recognisable with the use of a hand-lens and in merely polished slabs. 

 In most cases thin sections are necessary for its demonstration. In other cases, 

 the structure, though essentially the same as in the forms above mentioned, is more 

 minute. Thus in the common S. typica, Rosen, of the Wenlock Limestone, the 

 skeleton-fibre, as seen in thin sections, has a minutely dotted aspect (Plate I, fig. 

 3), the clear spaces in the fibre being very small, and often replaced by opaque dots. 

 That in all these cases the clear spaces in the fibre are really of the nature of 

 vacuities, filled with transparent calcite, can hardly be doubted ; and that these 

 vacuities are of the nature of vesicles rather than of tubes, would seem certain from 

 the fact that there is no sensible difference in their shape as displayed either in 

 tangential or in vertical sections. 



In the species of the genus Stromatoporclla, Nich., not only is the skeleton-fibre 

 similarly vacuolated, but the cavities in the fibre often assume the character of a 



