THE STEUCTDEE OF PORITES, 489 



In Montipora and in Porites, the calicle-walls fuse for tlieir 

 whole height, and the fossae are merely sunk into the common 

 reticular skeleton. In Montipora it is impossible to assign definite 

 portions of this reticulum to the individual calicles ; but 

 in many Porites this can be done, the surface being marked 

 off into polygonal areas, each area being in close contact with 

 those adjoining it, and with its surface often sloping inwards 

 towards the fossa in the centre. Hence it has been stated that 

 Montipora has a coeuenchyma, but Porites little or no coenen- 

 chyma (Milne-Edwards and Haime * ; see also quotation from 

 Klunzinger, ante, p. 487, footnote). An extended survey of 

 Porites shows that this distinction is quite artificial. On many 

 of the forms with these thick-walled calicles marked off into poly- 

 gonal areas, these areas become gradually invisible in the older 

 parts of the stock ; while forms in which no areas are traceable 

 at all, and in which the calicles are sunk straight into a reticular 

 skeleton as they are in Montipora, are quite common. Milne- 

 Edwards suggested that perhaps these lacter should be placed in 

 a new genus. This suggestion was carried out by Verrili f, and 

 further emphasized by Klunzinger, who placed Synaroea, Verr., at 

 the very end of the Poritidse, because it alone had a coenenchyma ; 

 whereas, as above stated, calicles marked off into areas and 

 calicles sunk in a level coenenchyma can frequently be found on 

 one and the same stock +• 



Passing in review the various walls found in Porites, we shall 

 see again how impossible it is to separate a group as a new 

 genus merely on account of the great thickness and level tops of 

 their walls. 



Thin Membranous Walls. — These walls have very different 

 appearances according as the intrathecal skeleton rises to the 

 level of the wall or sinks down below that level. In the former 

 case we have a surface like that shown in fig. 5, PL 35 ; 

 only here the skeletal elements are somewhat thickened. In 

 these cases it is common to find the edge of the wall incom- 

 plete, the calicles communicating freely one with the other. 

 These communications doubtless become perforations as the 

 stock thickens. 



In these thin-walled forms the wall itself is composed of a 



* Ann. Sci. Nat. 3rd ser., xvi. 1851, p. 24. 



t Bulletin Mus. Oomp. Zool. i. 1864, p. 42. 



\ For another supposed generic distinction between Porites and Synarcea 

 ee p. 49-4 footnote. 



