ON THE ASTROCGEXI^ OF THE STJTTON STONE, lOi 



8. 0)1 the AsTROCGENi^ of tli3 Sftton Stone and other Deposits of 

 the Infea-Lias of South Wales. By Prof. P. Maetin Duncan, 

 M.E. (Lond.), P.R.S., P.G.S. (Read J^ovember 4, 1885.) 



[Plate VIII.] 



A VERY well-preserved specimen of Astrocoenia gibhosa, nobis *, has 

 lately come to handwhicb was collected by the lateE. B. Tawney,Esq,, 

 F.G.S., and as it illustrates many interesting points in regard to the 

 morphology of the species, I have thought that a description of it 

 would be interesting to those Fellows of the Society who take an 

 interest in the Madreporaria of the Jurassic strata. 



The specimen is gibbous and worn on the free surface, and it has 

 been carefully cut transversely and polished at the distance of an inch 

 from the top, The form has grown vigorously in some parts and more 

 slowly in others, and there are quite young as well as old corallites 

 to be observed in the colony. Except in one or two places, fossil- 

 ization has not interfered with the perfect preservation of the 

 structural details, and the originally hard parts of the coral are 

 represented by dense white mineral, while the interspaces are 

 filled with more or less transparent, dark, carbonate of lime. 



On one side of the S])ecimen and on the upper surface close by, 

 are some corallites, which, as they are characteristically Astrocoenian, 

 may be noticed first of all. On the side of the colony the corallites 

 have been worn so as to show longitudinal views of their walls and 

 septa (PL VIII. fig. 1). The walls of adjacent corallites are united, 

 and it is only in one or two places that a very thin line of semi- 

 transparent calcite denotes that the fusion has been incomplete. 

 Elsewhere the walls of the corallites are perfectly united, the line 

 of junction being invisible. The width of the joined walls is small, 

 and it varies at diff'erent heights in the exposed portions ; some- 

 times the combined structure is as broad as one half, and in most 

 places it is about one fifth of the diameter of a cahce. These 

 measurements give the presumed width of the wall of each corallite 

 as one quarter, and as one tenth part of the diameter of the lumen 

 of the corallite. The walls are homogeneous and there is nothing 

 like exotheca or coenenchyma between them. 



The natural section shows _ that the corallites are of diff'erent 

 diameters and lengths, and that they are often slightly curved and 

 do not increase or diminish much in their course to the surface. 

 The septa seen in the longitudinal view are thin, slightly wavy 

 from above downwards, and sharply granular at their sides. There 

 is a small amount of endotheca. The walls can be traced to the 

 free ends of the corallites above, and to the sectioned corallite ends 

 at the polished surface below. It is evident that the coralHtes are 

 polygonal. 



* Monogr. Brit. Foss. Corals, ser. 2, pt. iv. No. 1. Pal. See. 1867, p. 18. 



