294 DESCRIPTION OF CHALK CORALS. 
dianthus, also the Flustra of Faujas de St. Fond, as well as the Retepora clathrata 
of Goldfuss, may possess similar essential constituents. It is therefore proposed 
to designate the chalk coral by the term Holostoma (odoc totus, ordua os), in allu- 
sion to the surface being wholly occupied by apertures to visceral cavities ; and 
in contradistinction to their limited distribution in Apsendesia. 
Holostoma, n. g. 
Tubular, branched, free except at the base ; tubes simple, of limited growth ; 
apertures distributed over the whole surface, progressively narrowed, ultimately 
obliterated ; external thickening slight. 
Holostoma contingens, n.sp. (Tab. XVIII. A. figs. 9 & 9a to 99.) 
Branches dichotomous, deep, thin, sides flat, back round, front slightly cari- 
nated with a discontinuous medial line; tubes in close contact arranged in 
opposite series, near the back parallel to that surface, elsewhere inclined towards 
the front ; apertures circular or polygonal, large adjacent to the front and some- 
times slightly raised ; on the sides and back of branches depressed, more or less 
bounded by a projecting wall, gradually contracted by a lamina extended from 
the sides, ultimately filled up. 
Five specimens of Hol. contingens were examined, and the finest is represented 
by figure 9; but not one of them exhibited a trace of a base or root. M. Miche- 
lin’s figure 4 a, of Aps. dianthus, exhibits a symmetrical radiation from a centre 
whence four branches issue; and the plane of growth is nearly horizontal. One 
of Mr. Dixon’s specimens displayed imperfectly a similar divergence ; but the 
stratum of ramifications undulated, without however any indication of an upward 
range or cyathiform mode of development ; and as other specimens agreed with 
it in this particular, the coral had also probably a tendency to a horizontal exten- 
sion. The manner of branching was essentially dichotomous (fig. 9), but shoots 
sometimes issued laterally ; and where great irregularities prevailed (fig. 9 a), it 
was not possible to detect any systematic bifurcations, the whole presenting an 
anastomosis not very unlike that of Faujas de St. Fond’s fig. 1 a (pl. 39). The 
ramifications were uniformly near each other, the successive subdivisions filling 
up the spaces which radiation would otherwise have produced ; and when a 
junction occurred, there was a limited union. The thickness of the branches 
varied very little; but the depth gradually increased from a perfect extremity, 
where it was merely an obtuse point, to fully a line and a half. The sides though 
flat were not uniformly even; and they occasionally exhibited transverse ridges 
of limited extent, due apparently to irregular protrusions of apertures ; and they 
