xlii BRITISH FOSSIL CORALS. 



indistinctly separated by superficial, narrow grooves. Columella not much developed at 

 its apex ; but having a tendency to become compact, and to fill up the visceral chamber 

 towards its basis. Septa very thick, closely set, scarcely granulated, and very feebly 

 denticulated. Dissepiments little developed. 



Typ. sp., Baryastrea solida, Milne Edw. and J. Haime, loc. cit. 



68. Genus Acanthastrea. 



Milne Edw. and J. Haime, Comptes rend., t. xxvii, p. 495, 1848. 



Corallum forming a slightly convex mass, with its upper surface strongly echinulate, and 

 its under surface constituting a plate, covered with a complete, thin epitheca. Gemmation 

 sub-marginal or marginal. Corallites united by their walls, which are somewhat cellular. 

 Calices sub-polygonal, with broad, spiniferous, simple, common margins. Columella rudi- 

 mentary or septal. Septa exsert, strong, and armed with projecting spiniform teeth, the 

 largest of which are situated near the walls, instead of being the central ones, as in the 

 preceding genera. Dissepiments very numerous. 



Typ. sp., Acanthastrea spinosa, Milne Edw. and J. Haime, Comptes rend., t. xxvii, p. 495. 



69. Genus Synastrea. 



Milne Edw. and J. Haime, Comptes rend., t. xxvii, p. 495, 1848. 



Corallum pediculate, and increasing in breadth more than in height. Gemmation 

 sub-marginal. Corallites intimately united by their walls. Calices superficial, distinct at 

 their centre, but not so towards their circumference. Columella very small. Septa con- 

 fluent, progressing from one calicular centre to another without interruption, exsert, and 

 hiding the walls, over which they extend ; their calicular margin almost horizontal, and 

 armed with nearly equal teeth. 



Typ. sp., Synastrea Savicjnyi, Milne Edw. and J. Haime, Ann. des Sc. Nat., 3 rae serie, vol. x, 

 tab. ix, fig. 12. 



70. Genus Thamnastrea. 



Thamnasteria {in parte), Le Sauvage, Mem. de la Soc. d'Hist. Nat. de Paris, vol. i, p. 241, 1822; Tham- 

 nastrea, ejusd., Ann. des Sc. Nat., l re serie, vol. xxvi, p. 328. 



Corallum having confluent septa, and most of the other characters of Synastrea, but 

 forming a fasciculus of columns or thick branches, erect, and of a more or less arborescent 

 aspect. 



Typ. sp., Thamnastrea dendroidea, Le Sauvage, Mem. de la Soc. d'Hist. Nat., vol. i, tab. xiv. 



71. Genus Goniastrea. 



Milne Edw. and J. Haime, Comptes rend., t. xxvii, p. 495, 1848. 



Corallum always increasing by successive fissiparity, and forming a convex or tabulated 

 mass, of a dense structure. Corallites intimately united from top to bottom by their walls, 



