DESCRIPTION OF CHALK CORALS. 279 
I. pinnata, considers the distribution of the ‘‘ cells” in one or more rows as not 
an essential difference, referring to Aulopora and Defrancia for examples of species 
with such variations. It is not advisable in this notice to inquire into the cha- 
racters of those genera, but it is evident that the existence of one or more rows 
of tubes should be studied in conjunction with the whole composition of the 
polype or its stony representatives, in attempting to ascertain a generic basis. 
2ndly. The rows of tubes in Idmonea range downwards exactly as they appear on 
the surface, constituting almost wholly the interior, being unaccompanied by any 
distinct structure. This feature is occasionally exhibited on natural surfaces’, 
and its existence was ascertained in Idm. cretacea, by purposely wearing down a 
fragment of a branch. On the contrary, the fasciculi in the fossil under con- 
sideration form, except in the very centre, only a portion of the branch, being 
surrounded by a reticulation connected with the external network. 3rdly. The 
precise mode of developing successive rows in Idmonea is not known to the 
author, nor was it detected in the small specimen just alluded to; but from M. 
Edwards’s statement and figures?, as well as from the characters of that fragment, 
the tubes of one row spring directly at the back of those of the row next inferior, 
allowance being made in some cases for lateral additions. In the fossil under ex- 
amination the fasciculi issue from the centre of the stem (Tab. XVIII. A. fig. 6 e), 
and after a limited upward growth blend into groups and incline outwards. The 
interior of Idm. truncata, or of the other four apparently allied fossils, is not de- 
scribed in the works consulted, but possibly no essential difference exists. 4thly. 
M. Milne-Edwards (loc. cit.) states, that the transverse rows of tubes do not, for 
the greater part, reproduce new branches ; and their very nature would, it is con- 
ceived, prevent such developments ; on the contrary, in the chalk fossil, the fas- 
ciculi are not unfreyuently enlarged and become true branches, near the base 
of only one of which two or three oral apertures appeared, not on the back, but 
lateraily. These branches were independent of terminal bifurcations. 5thly. The 
species of Idmonea described by MM. Lamouroux, De France and Milne-Edwards 
have no marked, foraminated structure on any part of the exterior, similar to the 
network exhibited in figure 66, Tab. XVIII. A. The reverse side of Id. cretacea 
(Tab. XVIII.A. fig. 5) is however boldly ribbed with intermediate rows of elon- 
gated foramina, which penetrate obliquely under some, if not all conditions ; 
1 Consult M. Milne-Edwards’s Illustrations, op. cit. pl. 12. f.3 ¢ (by error 4c); pl. 9. f. 2, &e.: also 
Goldfuss’s fig. of Idm. quadrata, pl. 9. fig. 15 f. Corrigenda, p. 244, art. Ret. disticha. 
2 Op. cit. Mém. sur les Crisies, preliminary Remarks to Idmonea; also pls. 9 & 12. 
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