40 BRITISH FOSSIL CORALS. 



2. Genus Holar^a (p. lvi). 



HolartEA Parisiensis. Tab. VI, figs. 2, 2 a. 



Alveolites Parisiensis, Michelin, Icoe. Zooph., p. 166, tab. xlv, fig. 10, 1845. 



Corallum composite, and appearing to have lived fixed to the stem of some Fucus, 

 which it incrusted all round, so as to constitute, after the destruction of this extraneous 

 body, a hollow cylinder, open at both ends. The lamellar expansion thus rolled round is 

 very thin, and its inner or basal surface is covered by an extremely delicate epitheca. The 

 calices which occupy the opposite surface, and are consequently placed all round the 

 exterior of the above-described cylinder, are infundibuliform, deep, irregularly polygonal, 

 surrounded by a prominent margin, and sometimes slightly turned towards one of the 

 extremities of the corallum, which was probably its upper end. The fossula is small and 

 circular ; its centre is occupied by a fasciculated columella, composed of delicate vertical 

 processes, which are quite separated from each other, excepting towards the apex (fig. 2 a). 

 The vertical section of the corallum, by means of which the composition of the columella 

 is seen, shows also that the tissue of the whole mass is uniformly and delicately spongy ; 

 no appearance of costse, of septa, or of any radiate structure is perceptible. The diameter 

 of the specimen that we have figured is about a line and a half, and the thickness of the 

 lamellar expansion that constitutes this cylinder, about half a line ; the calices are also 

 about half a line in breadth. 



This species has been found both in the London Clay at Barton and the Calcaire 

 grossier of the environs of Paris. The British specimen represented in our plates belongs 

 to the cabinet of Mr. Frederick Edwards. We have examined many of these fossils, but 

 owing to the very small size of the corallites, and the extremely delicate structure of their 

 constituent parts, we fear that some of their characteristic features may have escaped from 

 observation, and we feel much uncertainty respecting the natural affinities of the generic 

 division of which it is as yet the only representative. We have not been able to ascertain 

 the existence of any tabulae in the interior of the visceral cavity, and therefore it would 

 appear to be allied to Poritidse rather than to Milleporidae ; but it bears great resemblance 

 to the latter, and we are inclined to think that, when better-preserved and older specimens 

 become known, it will prove to be a tabulated Zoantharia, and if that be the case, there will 

 no longer be any reason for distinguishing Holarasa from our genus Axopora (p. lix). It 

 is therefore only provisionally that we place it in the family of the Poritidae. 



