DESCRIPTION OF CHALK CORALS. 277 
evidently been effected, as in many other Bryozoa, by local secretions through 
possibly, in this case, undetected pores (figs. 5a to 5e). 
The reverse surface was rounded (fig. 5f), and the ribs were generally very 
prominent except at the base of the aged specimens (fig.5). They were waved, 
often subdivided and occasionally anastomosed. The furrows were well-defined 
throughout the greater portion of the specimens, but near the base they became 
likewise less distinct. The foramina situated in the furrows were also strongly 
exhibited, forming single rows of elongated indentations, though in parts which 
appeared to have been injured during development they exhibited a perfect net- 
work ; and in a slightly worn down reverse surface (fig. 5 g) they assumed the 
character of minute apertures to microscopic tubuli, which penetrated upwards 
and inwards. 
Respecting the characters exhibited by the reverse surface one or two points 
deserve consideration. Lamouroux seems to have known only a single species 
(Idm. triquetra), and in his generic characters he says the reverse side is ‘‘ légere- 
ment canaliculée, trés-lisse et sans aucune apparence de pores.” (Expos. Métho- 
dique, p. 80.) According to M. Michelin that species has either free or attached 
branches'; adding however that “‘l’espéce en discussion est adhérent, par ex- 
ception, jusqu’a présent.” (loc. cit.) An American Idmonea* exhibited on the 
reverse side in some fragments a complete flattening, but the surface was un- 
evenly impressed ; in others, though the triangular form was retained, the out- 
line was slightly convex, and in one case partly flat, partly rounded, indicating 
an attached growth to a certain extent at least. Respecting the reverse side of 
Idm. radians, a decidedly free growing species, the author of these memoranda 
possesses no information ; it will however be shown in the next article that some 
fossils referred to the genus have on that surface a structure very similar to the 
one exhibited by Idm. cretacea ; but they are characterized by lateral fasciculi of 
visceral tubes, not transverse rows, a difference considered by M. Milne-Ed- 
wards’ a sufficient basis for a generic separation. Notwithstanding therefore a 
resemblance in the dorsal composition, M. Edwards’s determination respecting 
the present fossil is retained ; the species nevertheless being regarded as possessed 
of transition characters between Idmonea and the corals about to be noticed. 
‘ Teonographie Zoophytologique, p. 234. “I. ramosa, ramulis divaricatis, adhewrentibus vel liberis.” 
> Quarterly Journ. Geol. Soc. vol. i. p. 524-25. 
3 Edit. 1836 of Lamarck, t. ii. p. 283, art. Retepora truncata, &c. Recherches sur les Polypes, Mém. 
sur les Crisies, &c. p. 27. 
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