276 DESCRIPTION OF CHALK CORALS. 
alternate, separated medially by a variable, longitudinal interspace ; reverse sur- 
face strongly ribbed with intermediate, punctured furrows ; base for attachment 
an irregularly expanded layer. 
Idmonea cretacea, Milne-Edwards, Annales des Sc. Nat., 2nde série, Zool. 
tome ix. pl. 12. fig. 5-5 b, and explanation of plate ; or Recherches sur les Polypes, 
Mém. sur les Crisies, &c. Description of plate p. 45, 1838. 
Idmonea Dixoniana, Mantell, The Medals of Creation, vol. i. p. 284. Lign. 64. 
figs. 6 & 12, description, p. 287, 1844. 
The fossil represented in Tab. XVIII. A. fig. 5 is believed to be identical with 
that delineated by M. Milne-Edwards from a minute fragment found in English 
chalk, and with the coral subsequently figured by Dr. Mantell. Three very illustra- 
tive specimens were obligingly lent by Mr. Dixon from his own cabinet. The largest 
was upwards of an inch in height, and had a lateral expansion of 14 inch; the 
other two were smaller, but of great interest, as they afforded a base of attach- 
ment (figs. 5 & 5h), similar to that of the recent species, Idm. radians (M. Edw. op. 
cit. pl. 12. fig. 4) ; and presented also the characters of inferior, aged portions of 
the coral. They all fully proved a free vertical growth, with a gradual, but un- 
equal, lateral expansion, assuming thereby a fan-shaped and not bushy character. 
The mode of branching was essentially dichotomous, the bifurcations commen- 
cing almost at the base, and occurring invariably in the same plane. The dis- 
tance between the points of separation varied from half a line to 4 lines, the least 
intervals being generally near the base : the breadth of the youngest branches was 
about a quarter of a line, and that of the oldest half a line. 
The longitudinal interspace between the transverse rows of tubular apertures 
in what were regarded mature fragments, was sometimes indistinct, sometimes 
traversed by a ridge, and it occasionally exhibited a broad, flat area ; while in some 
aged fragments it agreed with the character represented in M. Edwards’s figure 5 a 
(loc. cit.). The breadth of the front of the branch, or the portion occupied by the 
tubular mouths, equalled that of the reverse side; but as the coral thickened the 
latter swelled out, and the free part of the tubes formed a kind of crest. The 
transverse rows rarely contained more than three apertures ; sometimes only 
two were visible ; and care was often necessary to distinguish more than one, the 
innermost being predominantly conspicuous, and masking the others. The whole 
of this surface, not occupied by tubular extremities, was apparently solid, under 
the magnifying powers employed in the examination, and smooth ; and the most 
aged fragments gave no indications of a fibrous thickening ; the additions having 
