CORALS FROM THE CORAL RAG. 97 
Genus THAMNASTREA, (p. Ixil.) 
THAMNASTREA ARACHNOIDES. Tab. XVIIJ figs. 1, 1a, 14, 1¢, 1d, le, I, Ig, UR, We, Wy TX, 
MADREPORA ARACHNOIDES, Parkinson, Org. Rem., vol. ii, p. 54, tab. vi, figs. 4, 6; and 
tab. vii, fig. 11, 1808. 
AsTREA approaching to A. aANNULARIS, Conybeare and W. Phillips, Geol. of England, 
p. 188, 1822. 
EXPLANARIA FLEXUOSA, Fleming, Brit. Animals, p. 510, 1828. 
ASTREA ARACHNOIDES, Hyusdem, loc. cit., p. 510. 
= — J. Phillips, Mustr. of the Geol. of Yorkshire, vol. i, p. 126, 1829. 
EXPLANARIA FLEXUOSA and AsTREA ARACHNOIDES, S. Woodward, Syn. Table of Brit. Org. 
Rem., p. 6, 1820. 
—— — Morris, Cat. of Brit. Fossils, pp. 31—36, 1843. 
SIDERASTREA AGARICTAFORMIS, M‘Coy, Ann. of Nat. Hist., s. ii, vol. 2, p.401, 1848. 
THAMNASTREA ARACHNOIDES, Milne Edwards and J. Haime, Polyp. Paleoz., ete., p. 111, 
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. 1, le); im other specimens it is composed of foliaceous 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 strize, 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 (fig. 1c), 
and produce the appearance observed on the specimens, which Prof. M‘Coy has described 
under the name of Stderastrea agariciaformis. When well preserved, the calice shows a 
slight annular elevation round the fossula, which is well characterised, but very shallow, 
and contains a co/umella composed of a various number of papille (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 
13 
