118 BRITISH FOSSIL CORALS. 



Genus Thamnastrea, (p. xliii.) 



1. Thamnastrea Lyelli. Tab. XXI, figs. 4, 4«, 45. 



SiDERASTKEA Lamouroxjxi, M'Coij, Ann. of Nat. Hist., s. ii, vol. ii, p. 419, 1848. (Not 



Thamnastrea Lamourouxi, Lesauvage.) 



We have seen but a few fragments of this fossil : some were very large, iiTegularly 

 cylindrical, and somewhat mammillose ; others were slender, and these differences must 

 have given to the entire mass a general aspect somewhat different from that of the T. 

 dendroidea (or T. Lamourouxi) found near Caen, in Normandy. In the latter the columnar 

 branches, which constitute the compound mass of the coral, appear to vary very small in 

 diameter, however large the size of this mass may be. The calices are very unequally 

 approximated, and where they are the less crowded in one direction most of the septa 

 assume a transverse direction. The fossula is not surrounded by a circular elevation cor- 

 responding to the w^all, but when the corallites have been worn down the latter becomes 

 visible, and, although very thin and feebly developed, shows that the radiate laminae are 

 formed by the costse as well as by the septa. The columella is small, but in general well 

 characterised and composed of one or two round papillse. The fossula is not deep, but 

 never quite superficial. The septa form three cycla, which are often complete, but some- 

 times those of the last cyclum are deficient in one or two of the systems. They are thin, 

 denticulated, rather closely set, not very exsert, and somewhat unequal alternately ; most 

 of them are flexuous towards the circumference of the corallites. Those of the second 

 cyclum differ but little from the primary ones, but are not quite so broad ; the tertiary ones 

 are much narrower and thinner ; they do not appear to incline towards each other, and 

 become united at their inner edge. In some well-preserved calices very distinct paliforra 

 lobules are placed between the columella and the septa of the first two cycla ; the primary 

 ones are narrower and more central than those corresponding to the secondary septa ; the 

 .latter do not occur in the systems where the tertiary cyclum is incomplete. Diameter of 

 the calices one and a half line. 



This fossil is found at Stonesfield, and is in the collection of the Geological Society 

 and of Mr. D. Sharpe. A specimen belonging to the Cambridge Museum was met with 

 at Minchinhampton, and a cast found near Bath, by Mr. Bowerbank, appears to belong to 

 the same species, although the calices are rather smaller and more crowded than in the 

 above- described specimens. 



Tliamnastrea Lyelli is very much like T. affims" and T. dendroidea (or T. Lamourouxi)^ ' 



^ Milne Edwards and J. Ilaime, Monogr. des Astrcides, Ann. des. So. Nat., s. iii, vol. xii, p. 158. 



® It is the same fossil that Lamouroux described under the name of Astrea dendroidea, Expos. 

 Method., pi. Ixxviii, fig. 6, and afterwards called by Dr. Lesauvage Thamnastrea Lamourouxi, Mem. de la 

 Soc. d'llist. Nat. de Paris, vol. i, tab. xiv. 



