166 BRITISH STROMATOPOROIDS. 



closeness of the reticulation, due in part to the coarse nature of the skeleton-fibre, 

 and in part to the proportionately small size and the irregular distribution of the 

 canals which traverse the ccenosteum. The skeleton-fibre is minutely porous (Plate 

 XXI, fig. 2), but in very many cases, where the skeleton has not been perfectly 

 preserved, the pores are represented by dark or cloudy dots only. 



Two i^rincipal groups of forms of 8. concentrica may be distinguished, to which 

 a third, of a more doubtful nature, may be provisionally added. 



The first group comprises what may be considered the normal form of the 

 species, in which the ccenosteum is spheroidal or irregular in shape, the latilamina? 

 are simply curved or undulated, and the surface is smooth aud without " mame- 

 lons." The astrorhizas (Plate XXI, fig. 3) are small, their centres being from 

 7 to 10 mm. apart, and are not surrounded by sheaths of concentrically disposed 

 laminae (" astrorhizal cylinders"). 



The forms of the second group may be included under the varietal name of 8. 

 concentrica, var. colliculata, Nich. In this variety the ccenosteum usually has the 

 form of a thick cylinder, with a bluntly conical apex (Plate III, fig. 5), composed 

 essentially of laminae rolled concentrically round a vertical line. The astrorhizaa 

 are comparatively small, but are developed in superimposed groups, and are 

 commonly the centres of more or less definite " astrorhizal cylinders," the spaces 

 between which are filled up by undulated and flexuous laminae. Owing to this 

 disposition of the astrorhizaa, the surface exhibits numerous eminences or " mame- 

 lons," which may be rounded or sometimes acuminate, or at other times more or 

 less drawn out in the direction of the long axis of the fossil (Plate III, fig. 5). 

 The minute structure of the skeleton in this variety does not differ in any recog- 

 nisable respect from that of normal examples of the species. 



To the above I may add, under the provisional name of 8. concentrica, var. 

 astrigera, Nich., a third group of forms distinguished essentially by the large size 

 of the astrorhizaa, the centres of which may be 2 or 3 cm. apart, while their 

 branches are comparatively few, and divide dichotomously at wide intervals 

 (Plate XXIV, fig. 10). The only examples of this form with which I am 

 acquainted occur in the Devonian Limestones of Devonshire, and their state of 

 preservation is, unfortunately, such that I can say nothing as to the general form 

 of the ccenosteum, or the condition of the surface. The best-preserved examples 

 of this form which have come under my notice exhibit a microscopic structure of 

 the skeleton which, except as regards the astrorhizae, appears to agree in all 

 essential respects with that of typical examples of 8. concentrica. In the worse- 

 preserved examples the skeleton has been more or less extensively replaced by 

 calcite and its canal-system filled up with calcai'eous mud, thin sections thus 

 appearing under the microscope in a " reversed" condition (Plate XXIV, fig. 9). 

 In thin slices of such examples the astrorhizal canals often show a singular 



