448 JR. F. Tomes — Inferior Oolite Madreporaria. 



a little more than half the length of those of the primary cycle, 

 while those of the fourth cycle are quite short. 



The dissepiments are very numerous ; stout, arched, and they 

 completely fill up the loculi. They continue so high up in them 

 that they are seen almost close beneath the denticulations of the 

 septa. The diameter of a large corallum is from three to four 

 inches, and the height about one inch. The diameter of the calices 

 is half an inch. Inci'ease takes place by gemmation in the interval 

 between the calices. 



Although allied to Platastrcea Conybeari from the Great Oolite, the 

 present species may be readily distinguished from it by its much 

 smaller size and regularly lenticular form. Again, the septa of 

 P. Conybeari meeting in the centre of the visceral cavity, are much 

 thickened quite low down in it, and these form a mass which, when 

 those parts of the septa which are above it have been worn down, 

 comes in view, and has so much the appearance of a columella as to 

 have led MM. Milne Edwards and Haime to place the species in 

 the genus Plerastrcea. Nothing of the kind is observable in Pla- 

 tastrcea endotliecata. 



I take the present opportunity of making a few remarks on Pla- 

 tastrcsa Conybeari. It is not rare at Cooinbe Down, from which 

 place the type-specimens were obtained. It is sometimes of great 

 size, and although, generally speaking, of a globose form, is often 

 extremely irregular in its outline, and the calices on some parts have 

 a corresponding irregularity in shape. "When these irregular calices 

 are worn down, they present precisely the appearance of those shown 

 in the figure of Clansastroza Pratti figured in the History of British 

 Fossil Corals. But the more regular calices of Isastrata Conybeari, 

 figured on the same plate, would never by any condition of fossili- 

 zation or any amount of wear resemble those of the then supposed 

 Clausastrcea. But indeed no one except Prof. Duncan has suggested 

 such a possibility. 



Of the type-specimen of Clausastrcea Pratti (afterwards Plerastrcea 

 Pratti) the original describers observe that it is so ill preserved that 

 they could not give a complete description of the species from it. 

 Notwithstanding this, no doubt seems to have crossed Prof. Duncan's 

 mind as to its fitness to furnish a satisfactory description, and he 

 has accordingly entered quite recently into its details. 1 I also have 

 lately examined the type-specimen, and can confirm what has been 

 said of its unfavourable condition by MM. Milne Edwards and 

 Haime. It is a young example and bears a very close resemblance 

 to many of the specimens from Coombe Down. The supposed papillose 

 columella observed in some of the calices is nothing more than the 

 blending of the septa in the centre of the calice, the denticulations of 

 which, resembling in shape those I have figured on the plate accom- 

 panying this communication, 2 are yet traceable in that part of the 

 calice which has been subjected to the least wear — that is to say, in 

 the centre. All other parts of the septa have been worn down quite 

 smooth. 



1 Quart. Journ. Geol. Soc. vol. xlii. p. 138. 2 Plate X. Figure 3. 



