CORALS FROM THE CORAL RAG. 95 



A horizontal section made some way down from the cahcular surface shows that the 

 walls remain simple, and very thin or even rudimentary, but the different corallites united 

 in a common mass are always very well delimitated, and the same septa never extend into 

 two adjoining visceral cavities, as might be supposed to be sometimes the case by the 

 aspect of the calices. No columella exists in most corallites, and in those where some 

 traces of a similar organ are met with they consist only in a few small trabiculae. It must 

 also be noted that although the small septa often bend somewhat towards the neighboming 

 larger ones, they always remain quite free at their inner edge, and that the calicular 

 gemmation takes place at a considerable distance from the centre of the calice. 



The British specimens of this species which we have examined had been found at 

 Steeple Ashton, Malton, and Hackness, and belonged to the collections of the Museum 

 of Practical Geology, the Geological Society, the Museums of Bristol and Paris, Mr. 

 Bowerbank, and Mr. J. Phillips. The same species is mentioned by Mr. Smith as 

 having been met with at Stanton near Highworth, Shippon, Bagley Wood Pit, Banner's Ash, 

 Well near Swindon, and Wilts and Berks Canal, South of Bayford. It is found also in 

 abundance at Lifol, in the departement des Vosges, Stenay, in the departement des Ardennes, 

 and at Heidenheini, in Germany ; specimens from these localities are in the collections of 

 the University of Bonn, the Paris Museum, M. Michelin, &c. 



The genus Isasirea, as already stated (p. 74), has been established for a certain number 

 of corals that we formerly placed in the genus Frionastrea (p. xli), but that differs from the 

 species considered as the types of this latter group, by the total absence or rudimentary 

 state of the columella, and by the corallites being separated only by a single mural lamina, 

 whereas as in Frionastrea they are independent of each other in their lower part, and 

 become intimately cemented together only near the calice. 



/. explanata is one of the best characterised species of this generical division. It 

 differs from /. limitata} I. Guettardana^ I. exjjlanulata,^ and /. Bichardsoni,* by 

 the size of the calices and the number of the septa. I. Mmisterana^ may be distin- 

 guished from it by the principal septa being thinner outwards, and becoming somewhat 

 thicker towards the two thirds of their breadth inwards. In /. polijgonalis^ I. oblonaa^ 

 and /. Michelini^ the walls are much thicker. In /. Jamellosissiwa^ the sej)ta are more 

 distant, and in /. tenuistriata}^ their number is twice as great. In /. Conyhearii} the 

 septa are, on the contrary, less numerous, and in /. seriali^"" they are very unequal and 



' Tab. xxiii, fig. 2. 



2 Frionastrea Guettardiana, Milne Edwards and J. Haime, Ann. Sc. Nat., 3d ser., t. xii, p. 137. 



3 Tab. xxiv, fig. 3. * Tab. xxix, fig. 1 



^ Frionastrea Munsteriana, Milne Edw. and J. Haime, loc. cit., p. 136. 



^ Astrea pohjgoiialis, Michelin, Icon., pi. iii, fig. 1. 



7 Tab. xii, fig. 1. 



8 Montastrea Michelini, Blainville, Diet. Sc. Nat., t. Ix, p. 339. 

 ^ Astrea lamellosissima, Michelin, Icon., pi. vi, fig. 1. 



10 Tab. XXX, fig. 1. 11 Tab. xxii, fig. 4. 12 Tab. xxi?, fig. 2. 



