490 ME. H. M. BEENARD OJT 



single ring of sjnapticular bars or plates, aud these may be 

 ai'ranged either zigzag round the calicle or in straight lines. 

 In the latter case, the lines form tbe sides of polygons. These 

 straight walls are, I expect, secondary; for, regarding the reticular 

 wall as primitive, the zigzag wall wotild be the more natural 

 derivative of such a reticulum. The zigzag line is formed by 

 single synapticular bars joining the alternating costal edges of 

 adjacent calicles. All stages of the straightening of this line 

 can be found. It is the straight wall which rises above the 

 surface as a thin membrane-like edge, and, in extreme forms, 

 may give the whole surface an alveolate appearance. The 

 whole intrathecal skeleton may remain deep down in the base 

 of the membranous pits, or septal striae may slightly thicken 

 the walls and serrate their edges. 



Reticular Walls. — Of these there is an immense variety, 

 and again the appearance is very different according as the 

 intrathecal skeleton rises to the level of the wall or is sunk down 

 below that level. They are thick and thin, round-topped or with 

 a sharp median ridge ; or again, if the calicle-depressions are 

 cylindrical, the intervening angles may be thick and reticular, 

 while at the points where the calicles touch one another, the 

 wall may be reduced to a single lattice-work. Of the thick walls, 

 we have already referred to the variety in which straight median 

 ridges mark off in polygonal lines the areas belonging to each 

 calicle. These areas may sink inwards funnel-shaped towards 

 their central fossae. Or, again, they may be quite level. In 

 this case, the median ridge frequently disappears, and we have 

 the coenenchymatous group (formerly Synarcea, Yerrill). The 

 most interesting section of these is formed by those in which 

 the thick walls rise up secondarily into ridges or papillae (fig. 6, 

 PL 35), very similar to those found in Montipora. This is an 

 interesting case of similar specialization arising under similar 

 conditions. But, in Montifora, the thick reticular walls are 

 characteristic of tbe genus : hence these coenenchymatous 

 developments on the tops of the walls are far richer and more 

 varied than they are in Porites, in which the thick reticular 

 wall is confined to a group only. I have so far found no traces 

 of the ' tuberculate ' specialization * of the ccenenchyma which is 

 so widely developed in Montipora. 



* Cf. Brit. Mus. Madrep. iii., Introd. p. 9 : also for figures, Ann. & Mag. 

 Nat. Hist. XX. (1897) p. 117, pi. ii. 



