DESCRIPTION OF CHALK CORALS. 317 
or within the edge ; but in the fossil under examination, a perfect exterior is 
convex, and the lamina composing it springs from the margin of the boundary, 
and so totally conceals the side wall, that adjacent complete cells exhibit shield- 
like discs surrounded merely by a furrow. M. Milne-Edwards' has besides re- 
ferred Cellepora Velamen and C. bipunctata to Discopora as limited by himself ?. 
One of the essential characters of that genus is, however, an almost or entire 
obliteration of the outline of the cells, so that the free surface of the coral ‘‘ ne 
présente que de faibles ondulations dans les lignes correspondantes a leur sou- 
dure, et que la position de ces loges n’est guére indiquée que par leur ouver- 
ture’;” on the contrary, in the fossil forming the subject of this notice, such a 
condition was detected only where irregularities prevailed. M. De Blainville’, 
again, includes Cellepora bipunctata among his extinct Membranipore, but in re- 
cent types of that genus the exterior is never wholly composed of a calcareous 
layer; and whatever may have been the mature condition of that fossil, the 
chalk coral had undeniably an earthy lamina which extended over the whole 
area of the cell. The foregoing statements, although very limited with reference 
to component structures, justify it is believed the inference, that Mr. Dixon’s 
zoophyte is not a Marginaria, a Discopora, or a Membranipora. 
The characters of Escharina (M. Edw. op. cit. p. 230) are, cells more or less 
calcareous, regularly and horizontally arranged in incrusting layers, and partly 
superimposed or not; surface convex ; boundary always visible and without a 
projecting edge, aperture lateral rather than terminal ; and it may be added, that 
in immature and abraded states, the surface of the cells presents open areas, 
bounded by the edge of the side wall; while in some species an irregular furrow 
occurs similar to that of Marginaria. On submitting the chalk fossil to the test 
of these characters, the cells will be found to be horizontally situated, forming a 
single layer, to have a convex surface when perfect, and in that condition to be 
bounded only by a furrow ; while immature or abraded exteriors display closely 
analogous appearances to those mentioned above: and to the extent observed, 
the circumference of the cells was not obliterated in aged states by external ad- 
ditions. These points of agreement might be regarded as constituting a suffi- 
cient approximation towards a generic identity ; the manner however of develop- 
ing the outer, calcareous lamina remains to be ascertained, especially when studied 
' Lamarck, ed. 1836, t. ii. p. 253. +16 & +18. * Ibid. pp. 218, 247. 
* Consult Goldfuss, tab. 36. f.11 (Cell. annulata), for one of the fossils assigned to the genus by 
M. Edwards, Lamk. t. ii. p. 253, 15. * Man. d’Actinologie, p. 447. 
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