100 BRITISH STROMATOPOROIDS. 



while the third seems, so far, not to have been detected in Britain. One of the 

 forms in question I am disposed to regard as probably the Stromatopora (Tragos) 

 capitata of Goldfuss, which, again, not improbably, may be identical with the 

 Parallelopora Goldfussi of Bargatzky. The other two forms have the typical 

 cylindrical or fasciculate coenosteum of the genus, and I shall speak of them by the 

 provisional names of I. Roemeri and I. oculatum ; since the d escriptions of Winchell 

 and Quenstedt are not sufficient to allow of any comparison of these with the two 

 described American species. 



Taking Idiostroma Roemeri (PL IX, fig. 6), of the Rhenish Devonian, as a 

 typical example of the genus, the coenosteum has the form of a generally branched 

 cylindrical stem, rooted basally to some foreign object, which it may partially 

 encrust. The stems vary from one to three centimetres in diameter, and they are 

 sometimes so far confluent as to give rise to a sub-massive skeleton, in which the 

 component cylinders are, however, still clearly l'ecognisable. Both transverse and 

 longitudinal sections (PL IX, figs. 7 and 8) show that each stem is traversed by a 

 main axial tube, which is intersected by numerous transverse, vesicular, or funnel- 

 shaped " tabulae." This axial tube gives off lateral branches, which are also tabu- 

 late, and which ascend towards the surface, giving off secondary branches in their 

 course. Sometimes there is more than one longitudinal tube, in which case the 

 central one is the largest, and the subordinate tubes run parallel with it at a little 

 distance. Whether the lateral branches given off from the main tube open on the 

 surface by definite apertures, or whether the latter has an opening at the end of 

 the stem, is in this species difficult to decide positively. Some of my specimens do 

 not show any openings, except the minute apertures of the ordinary zooidal tubes ; 

 but others exhibit here and there much larger perforations, which can hardly be any- 

 thing else than the apertures of the lateral branches of the axial tube. These 

 large apertures are often placed upon prominent elevations or "mamelons." 

 There is no reason, so far I can see, for doubting that the large tubes above men- 

 tioned must have had definite surface-apertures, though these may not be visible 

 in all specimens. 



The surface of Idiostroma Roemeri is highly characteristic, and is covered with 

 vermiculate ridges, formed by the confluence of rows of pointed tubercles, and 

 separated by deep winding grooves (Plate IX, fig. 9). Often these ridges radiate 

 from the apices of conical " mamelons," and in the intervals between them are seen 

 the circular openings of the ordinary zooidal tubes. Thin sections show that the 

 coenosteum is built up of numerous concentrically-disposed layers, which grow as a 

 series of deeply convex caps round the free end of the stem, where each layer is 

 thicker than elsewhere. The skeletal tissue is in the main reticulate, but the 

 confluence of the radial pillars and their horizontal connecting-processes is not 

 nearly so complete as in the Stromatoporidce. Hence, not only are the radial 



