280 DESCRIPTION OF CHALK CORALS. 
but it differs in not being thickened, and in the portion near the base having 
to the extent observed no external prolongations of the pores. Among the corals 
more recently referred to the genus, but believed to possess structures allied to 
those of the English chalk fossil, the exterior seems to vary. Idm. semicylindrica 
is said to have “‘ fein punctirte stammchen ”’ (Roemer, op. cit. p. 20), and the 
inner side is shown to be distinctly punctured ; Roemer does not allude to the 
outer composition of dm. pinnata beyond the observation respecting the oral 
apertures ; but the fossil specifically identified with it by M. Michelin, “‘ has one 
face almost smooth, and the other very porous'.” Idm. aculeata is described as 
having the greater portion covered with little waved striz*, and the delineations 
show also a foraminated reverse surface at least ; dm. tetragona is striated and 
very porous on the under part*; respecting the exterior of Idm. truncata nothing 
is mentioned by Prof. Goldfuss. Thus in every species but the last a network 
occurs, though varied in its subordinate details, and possibly in the exceptional 
case it is not wanting. 
The differences noticed in the preceding paragraph, taken in the aggregate, 
mark, it is conceived, an essential distinction between the polype which de- 
veloped Idm. triquetra with the other allied corals, and that which formed the 
chalk fossil (Tab. XVIII. A. fig. 6), as well as its apparent analogues ; regarding 
the portions commonly called the polype as only the visceral and oral structures 
of the animal. In all the five polyparia so repeatedly alluded to, an identity of 
composition moreover is evident ; and the variations in the minor characters de- 
pictured as well as described by the authorities quoted, prove the existence of 
distinct species. The fasciculus of tubes partakes also of this want of uniformity, 
and therefore supports the inference that it is not a mere specific peculiarity. 
A similar unity of leading components is equally manifest in true [dmonee ; and 
if in Idm. cretacea a structure is superadded to meet the wants of a free, up- 
ward growth, still being associated with the most important elements of typical, 
attached species, its existence should be regarded as indicative rather of a sub- 
group, than as a warranty for placing in the genus certain corals possessed of 
such a character, but differing markedly in other prominent particulars. 
No doubt, it is presumed, can be entertained of the coral having been formed 
by an ascidian polype, or that it belongs to the family Tubuliporide, as at pre- 
sent constituted. 
‘ Teonog. Zoophytol. p. 203. pl. 52. fig. 9a, 9 6. > Ibid. p. 203-4, pl. 52. f. 104, >. 
* Ibid. p. 219. observ. pl. 53. f.19 0b. 
