278 DESCRIPTION OF CHALK CORALS. 
Possibly when more is known of the genus, it will be found advisable to propose 
two sections or subgroups, one for creeping, affixed species, the other for those 
which have a free vertical growth. 
Tab. XVIII. A. figs. 6 & 6a to Ge. 
The fossil next to be noticed consists of free branches springing from a root, 
and characterized by two rows of protuberances the free extremities of fasciculi 
of visceral tubes : the surface of the branches presents a variable network and 
the interior is formed of downward extensions of the fasciculi, which compose 
the direct centre, also of a reticulated structure, the inward representative of the 
surface network. The visceral tubes open, so far as was ascertained, chiefly on 
the back of the fasciculi. 
Five corals possessing characters more or less allied to those represented by 
figure 6, Tab. XVIII.A., have been described by different authorities :—namely, 
1. Retepora truncata of Prof. Goldfuss', removed to Idmonea by M. De Blain- 
ville*; 2. Idmonea semicylindrica ; and 3. Idm. pinnata of Herr Roemer*; 4. Idm. 
aculeata, with 5. Idm. tetragona of M. Michelin’. It is unnecessary to offer any 
remarks respecting Prof. Goldfuss’s determination, the fossil delineated in the 
‘ Petrefacten’ being evidently not a Celleporida. It is stated in the preceding 
notice on Idm. cretacea, that M. Milne-Edwards” objects to the altered assignment 
of Ret. truncata, and he is of opinion that it should form the type of a particular 
division, intermediate between Idmonea and Frondipora. Respecting the former 
of those genera, the leading distinctions between Lamouroux’s® original species 
Idm. triquetra, as well as those subsequently described by M. De France? or M. 
Edwards, and the fossil under examination, appear to be,—Ist. that in the former 
the visceral tubes are arranged in transverse rows, the apertures having a similar 
disposition ; whereas in the latter they constitute round fasciculi, the mouths 
opening, to the extent known, in two rows or irregularly on the back of the 
protuberances. Herr Roemer (Joc. cit.), in his remarks on Idm. semicylindrica and 
' Petrefacten, p. 29. pl. 9. fig. 14, a Maestricht fossil. * Manuel d’Actinologie, p. 420. 
* Verst. Norddeutsch. Kreidegebirges, p. 20. tab. 5. f. 21 and p. 20. tab. 5. f. 22; also Michelin, 
Iconographie Zoophytologique, p. 203. pl. 52. fig. 9 a, b. 
* Ieonog. Zoophyt. p. 203. pl. 52. fig. 10; and p. 219. pl. 53. fig. 19. 
* Lamarck, Anim. sans Verteéb., edit. 1836, tome ii. p. 283; also Annales des Se. Nat., 2nde série, 
Zoologie, tome ix. Mémoire sur les Crisies, &c. or Recherches sur les Polypes, &e. 27. 
° Exposition Méthodique, p. 80. pl. 79. figs. 13-15. 
7 Dictionnaire des Sciences Naturelles, tome xxii. pp. 564, 565. 
