9S BRITISH FOSSIL CORALS. 



amounts often to twenty-six, twenty-eight, or even thirty-two. They are very closely set ; 

 their upper edge is almost horizontal, and delicately denticulated ; their size differs but 

 little (especially amongst those of the first two cycla), and most of them extend in an 

 almost straight line from the fossula of one calice to that of the neighbouring one. The 

 secondary septa in general bend towards the primary ones near their extremities, and often 

 become united to them by their inner edge (fig, \f). The size of the calices varies some- 

 what in the same specimen, but presents much greater variations in different specimens ; 

 their diameter varies from two to three lines, and even more ; the distance between the 

 calicular fossulse is in general three lines, but sometimes four lines. 



The aspect of this Coral differs very much, according to its mode of fossilization and 

 state of preservation. Thus when the upper part of the sejjta has been broken down to a 

 certain extent, as is often the case with the specimens found at Steeple Ashton, the calices 

 appear deep, almost polygonal, and much like those of Isastrea (see figs, ly and \k). It is 

 owing to a change of this kind that M. Michelin was led to suppose that a species very 

 nearly allied to this, and found at Le Mans, in France, was composed of two distinct Corals, 

 the one much resembling the specimen represented in fig. \(/, and the others extremely 

 thin, enveloping the first, resembling fig. \j, and "disappearing when rubbed with a hard 

 brush," that is to say, when the delicate terminal portion of the septal apparatus had been 

 worn away by the operator and the subpolygonal walls of the corallites denudated. The 

 various appearances here alluded to sometimes exist on different parts of the same 

 specimens, 



Tliamnastrea arachno'ides is very common in the Coral rag of Steeple Ashton, and is met 

 with also at Up ware, near Cambridge; atMalton; and, according to Parkinson, at Chatelor. 

 We have seen numerous specimens of this species in the collections of the Museum of 

 Practical Geology, of the Geological Society, of the Bristol, Cambridge, Paris, and Bonn 

 Museums ; and of Messrs. Bowerbank, Walton, Phillips, d'Archiac, and Michelin. 



The genus Thamnastrea was established in 1822 by Dr. Lesauvage, for a dendroid 

 Coral found near Caen, and it was on account of the general form of this fossil that it was 

 thus distinguished from the other Astreida^. In the Introduction to this Work, as well as 

 in our Monograph of the family of Astreidge, we adopted this genus, and assigned to it 

 characters furnished by structural peculiarities, which appeared to warrant its separation 

 from our genus Synastrea, as well as from the other divisions of the same tribe. But 

 having been enabled of late to examine some more perfect specimens of Lamom'oux's 

 Tliamnastrea, we have ascertained that the differences between these and our Synastrea are 

 not by far as great as we at first supposed ; thus the septa are in reality dentate in the first 

 as well as in the latter, and the columella varies almost as much in different calices of the 

 same species as from one species to the other, being, in some, composed of only one styliform 

 tubercle, and in others of two, three, or more papillae ; the general form of the compound 

 mass is evidently not here a character of generical value ; we, therefore, deem it advisable 



