98 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. 1f). The size of the calices varies some- 
what in the same specimen, but presents much greater variations im different specimens ; 
their diameter varies from two to three lines, and even more; the distance between the 
calicular fossulz 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 seyfa 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 Jsastrea (see figs. lg and 14). 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. 1g, and the others extremely 
thin, enveloping the first, resembling fig. 17, 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. 
Thamnastrea arachnoides is very common in the Coral rag of Steeple Ashton, and is met 
with also at Upware, near Cambridge; at Malton; 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 Zhamnastrea 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 Astreidse. In the Introduction to this Work, as well as 
in ow Monograph of the family of Astreidee, 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 Lamouroux’s 
Thamnastrea, we have ascertained that the differences between these and our Syzastrea are 
not by far as great as we at first supposed ; thus the sepfa 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 papille; the general form of the compound 
mass is evidently not here a character of generical value ; we, therefore, deem it advisable 
