FA VOSITIDJE. 31 



forms of Packypora, Lindstr.) the corallites are very much 

 compressed and flattened in form, becoming oval or sub- 

 triangular in shape. In other cases, again, the corallites are 

 partly contiguous and partly free (Vermipora, Hall, Romin- 

 geria, Nich., Chonostegites, E. and H., &c.), and in these 

 cases the corallites are prismatic where in contact, and more 

 or less cylindrical where free. In all the Favositida, however, 

 the corallites of the colony are in contact in some portions of 

 their extent. Wherever contact occurs, the walls of the coral-r 

 lites enter into a certain degree of union, though the extent of 

 this varies in different cases. In most of the Favositidce (in- 

 cluding all the typical forms) thin sections show that the 

 corallites always retain their proper walls, however closely 

 amalgamated they may appear to be ; so that the boundaries 

 of contiguous tubes are distinctly marked out by conspicuous 

 dark lines. Hence, also, weathered or fractured surfaces 

 usually (though not always) show the outsides of the coral- 

 lites. In one genus only (Nyctopora, Nich.) the fusion of 

 the walls of contiguous corallites appears to be complete. 



As a general rule the walls of the corallites are not specially 

 thickened ; but in certain types (Packypora, Lindstr., Striato- 

 pora, Hall, Trackypora, E. and H., and Ccenites, Eichw.), the 

 visceral chambers of the polypes become contracted by the 

 secondary deposition of dense sclerenchyma in concentric 

 lamellee, the amount of this deposit increasing as the calices 

 are approached. In Stenopora, Lonsd., this thickening of 

 the tubes takes place periodically, and the terminal portions 

 of the corallites become thereby periodically annulated on 

 their outer surface, the intervening non- thickened portions 

 being not in actual contact. In some cases this thickening 

 of the walls of the corallites takes place to such an extent 

 (as in Trackypora, E. and H.) as to give rise to all the 

 appearances which would be produced if the tubes were 

 embedded in a dense " ccenenchyma ; " and the existence 

 of such a structure has often been affirmed. Microscopic 

 sections, however, show that the calcareous tissue separating 



