14S BRITISH STROMATOPOROIDS. 



the centre of an inch or more. The upper surface is irregularly undulating, with- 

 out " inainelons," and exfoliating concentrically round the elevated points. Small, 

 but well-developed astrorhiza?, the centres of which are from 3 to 5 mm. apart, are 

 present ; but the central canals of these do not open on the surface by prominent 

 apertures. 



As regards internal structure, the ccenosteum is made up of closely concentric 

 lamina?, which are only slightly or not at all undulated, and of which from nine to 

 eleven in general occupy the space of 1 mm. The lamina? are minutely crumpled, 

 so as to give rise to more or less perfect radial pillars ; but these structures are 

 invariably confined to their respective interlaminar spaces, and are, therefore, not 

 " continuous." In this way the interlaminar spaces become broken up into 

 minute, often imperfect, lenticular cells (Plate XVII, figs. 10 and 12), which are 

 formed by the curved inflections of the concentric lamina? conjoined with the more 

 or less incomplete radial pillars. In general from eight to ten interlaminar spaces 

 occupy the space of 1 mm. The cells formed by the radial pillars or by the 

 bendings of the lamina? are very variable in size, but are mostly from ■§■ to yq mm. 

 in length. 



Obs. — C. vesiculosum is the type-species of the genus Clatkrodictyon, Nich. and 

 Mur. As in all the species of the genus, therefore, the radial elements of the skeleton 

 are incompletely developed, and never extend from one interlaminar space into 

 adjoining ones. While the radial pillars are thus imperfect, the concentric lamina? 

 of the ccenosteum are minutely undulated, and the interlaminar spaces thus become 

 broken up and subdivided into vesicles, the size and shape of which are exceedingly 

 variable. Hence vertical sections of G. vesiculosum (Plate XVII, figs. 10 and 12) 

 show a minutely vesicular structure, the ccenosteum appearing to be made up of 

 exceedingly small lenticular cells, arranged in horizontal or slightly curved rows. 

 On the other hand, tangential sections (Plate XVII, figs. 11 and 13) show the cut 

 ends of the isolated and imperfect radial pillars, together with the irregularly 

 divided edges of the concentric lamina?. The radial pillars, as seen in tangential 

 sections, show no traces of radiating " arms " or connecting-processes, but they 

 may sometimes become conjoined in short vermiculate rows. Small astrorhiza? 

 are seen in tangential sections, and these usually are arranged in vertical groups, 

 and have wall-less axial canals, but the openings of the latter on the surface are 

 not prominently elevated. 



C. vesiculosum, Nich. and Mur., belongs to a series of forms, the specific deter- 

 mination of which is exceptionally difficult. The forms in question, viz. G. vesi- 

 culosum, Nich. and Mur., G. variolare, Rosen sp., C. Linnarssoni, Nich., and C. 

 crassum, Nich., are all easily recognised as distinct when typical examples are 

 selected for examination. It is, therefore, expedient to distinguish them by 

 separate specific names. At the same time, there are close relationships between 



