78 TABULATE CORALS. 



thick-walled species which have been reg-arded as dendroid 

 forms of Favosites will more appropriately find a place in 

 Pachypora; and the same may be considered as likely as 

 regards the majority of the thick - walled types which have 

 been referred to Alveolites. 



The mere thickening of the walls would not of itself afford a 

 sufficient ground for the separation of Pachypora as a generic 

 division, since it is present in other groups ; but there are other 

 distinctive characters to be taken into account as well. Little 

 stress can be laid upon the external form of the corallum, but 

 all the known species are dendroid or frondescent. P. lamelli- 

 cornis, Lindst., the type-species, forms flattened branches, which 

 are often coalescent ; P. Fischeri, Bill, and P. frondosa, Nich., 

 grow as broad undulating expansions or fronds ; P. cristata, E. 

 and H., and P. cervicornis, De Blainv., are essentially ramose, 

 usually with cylindrical branches, but sometimes sublobate. 

 In all these cases the corallum is fixed to some foreign body 

 by its base ; and there is no epitheca, the whole of the free 

 surface being covered by the open mouths of the corallites. 

 The calices are sometimes circular, sometimes polygonal, some- 

 times markedly triangular ; so that the genus includes forms 

 which would, in this respect, fall on the one hand under Favo- 

 sites, and on the other hand under Alveolites. The calices are 

 in all instances more or less remote from one another, and 

 thin sections show that the cause of this remoteness is to be 

 found in the thickening of the walls of the corallites by a 

 secondary deposit of sclerenchyma. This thickening affects 

 the corallites throughout their entire length, but it is least 

 developed in the central and interior portion of the corallum, 

 and becomes much more conspicuous as the mouths of the 

 tubes are approached. Hence in tangential sections taken 

 close to the surface the visceral chamber is seen to be sur- 

 rounded by a dense calcareous envelope composed of numer- 

 ous delicate concentric laminae, and surrounded externally by 

 a clearly distinguishable proper "wall" (see PI. IV., figs. 2 a and 

 3 c). The tabulae are poorly developed, being few in number, 



