DESCRIPTION OF CHALK CORALS. 295 
were regarded as belonging to an exceptional rather than an essential structure. 
The crest or front of a branch (fig. 9 b) consisted generally of a congeries of fully 
open tubes variable in size and form, a few of them occasionally projecting and 
assuming a transversely linear arrangement: the medial line was by no means 
persistent or conspicuous. The whole of the specimens examined had the front 
imbedded in the matrix, and it is therefore impossible to state what changes dif- 
ferent conditions of growth might present ; while a portion purposely exposed, 
as well as the fragments which were detached, exhibited no indications of a final 
cessation of development. Immediately adjacent to the front, the apertures were 
also fully open, with more or less of irregularity in the general surface (fig. 9 ¢) ; 
but the sides of the branches exhibited for the greater part the characters shown 
by figure 4, or a network of ridges, the place of the meshes or apertures to the 
cavities being represented by a shallow depression with a minute, central foramen ; 
though in some cases no opening whatever could be detected. The back of the 
branches (fig. 9d) displayed similar structures, only the concavities were often 
arranged in rows, and the ridges assumed in consequence partly the semblance 
of longitudinal ribs. As respects internal composition, figure 9 e exhibits the 
characters of a slightly worn-down lateral surface, or obliquely intersected tubes 
varying in size from a minute pore to the mature dimensions ; and figure 9/f, a 
fragment abraded nearly to the centre of the branch, so as to expose the range 
of the cavities previously to assuming an outward inclination. In both cases the 
tubes adjacent to the back are nearly parallel to it, opening in that surface, 
and the outward or horizontal extension of the branches was due to them ; 
whereas the tubes composing the greater portion of the ramifications are more or 
less shown in each figure to range obliquely upwards, the successive young de- 
velopments giving the branches their great depth. Fig. 9g displays the general 
appearance of a vertical cross section, or tubes diverging outwards with a medial 
plate ; but the characters varied in every case examined, and the plate was some- 
times scarcely detectable. 
Diastopora ramosa, Michelin. (Tab. XVIII. B. figs. 1, 1a, 16.) 
Incrusting, dichotomously branched ; branches very variable in breadth, when 
in contact anastomosed ; apertures slightly projecting, irregularly distributed in 
two or more series. 
Michelin, Icon. Zoophy. p. 203. pl. 52. fig. 3a, 3b. Neighbourhood of Mans, 
Depart. Sarthe, and of Ciply, Belgium, 1840-1847. 
2Q2 
