CORALS FROM THE CORAL RAG. 97 



Genus Thamnastrea, (p. Ixii.) 



TuAMNASTREA AUACHNoiDEs. Tab. XVII, figs. 1, Ici, \h, \c, Id, \e, \f, ly, III, \i, y, 1/?-. 



Madkepora arachnoiues, ParJdnson, Org. Rem., vol. ii, p. .54, tab. vi, figs. 4, 6 ; and 



tab. vii, fig. 11, 1808. 

 AsTREA approaching to A. annularis, Conybeare and JJ'. Phillips, Geol. of England, 



p. 188, 1822. 

 ExPLANARiA FLEXUOSA, Flemivij, Brit. Animals, p. 510, 1828. 

 AsTREA ARACHNOiDES, Ejusdem, loc. cit., p. 510. 



— —J. Phillips, Illustr. of the Geol. of Yorkshire, vol. i, p. 126, 1829. 



ExPLANARiA FLEXUOSA and AsTREA ARACHNOIDES, S. Woodivard, Svn. Table of Brit. Org. 



Rem., p. 6, 1820. 

 — — Morris, Cat. of Brit. Fossils, pp. 31—36, 1843. 



SiDERASTREA AGARiciAFORMis, M'Coij, Ann. of Nat. Hist., s. ii, vol. 2, p. 401, 1848. 

 Thamnastrea akachnoides, Milne Edwards and/. Haime, Polyp. Paljeoz., etc., p. Ill, 



1851. 



Corallum composite, massive, and varying in its general form, but appearing in most 

 instances to have been fixed by the central part of its under surface, and to have spread out 

 as it grew up (fig. \,\c) ; in other specimens it is composed of foUaceous expansions, 

 which are sometimes superposed, so as to produce thick subdiscoidal masses, more or less 

 lobated towards the margin (fig. 1). The upper surface of these corals is in most specimens 

 slightly convex, but is sometimes very strongly so, or on the contrary quite flat, or even 

 concave, and by the figures given in Parkinson's Work it appears that there are in other 

 instances foliaceous lobes arising from it. In some young specimens the general form is 

 regularly turbinate. 



The basal plate is somewhat lobated, and presents some transverse swellings, which are 

 produced by a certain intermittance in the progress of growth. The epitheca appears to be 

 rudimentary, and the costal striae, which are very distinct, are straight, regularly crenu- 

 lated, very narrow, of equal size, and very closely set. The calices are shallow, unequally 

 developed, and vary considerably in their degree of approximation ; in general they are 

 rather closely set, and when most so, often become arranged in concentric series j^fig. \c), 

 and produce the appearance observed on the specimens, which Prof. M'Coy has described 

 under the name of Siderastrea agariciaformis. When well preserved, the calice shows a 

 slight annular elevation round the fossula, which is well characterised, but very shallow, 

 and contains a columella composed of a various number of papilla (two to eight). The 

 septa form three complete cycla, and in the largest calices they also represent an incom- 

 plete fourth cyclum in one half of one or two systems, so that their total number 



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