354 MR. GEORGE BROOK ON THE 



as to the interrelationships of the genera Madrepora, Turlinaria, 

 Montijpora, and Porites. The on]}'' essential distinction between 

 tfie families Madreporidse and Poritidse, according to the diag- 

 noses given by Duncan, is to be found in the condition of the 

 septa ; in the Madreporidse these are stated to be lamellar, 

 slightly porous, or solid ; in the Poritidse never completely 

 lamellar and often trabecular. This distinction may be satis- 

 factory so far as Turlinaria and Porites are concerned, but 

 would probably not have been regarded by Duncan as the most 

 important difference between the two genera. In my experience 

 the distinction does not hold good for all species of Madrepora 

 and Montipora. In Madrepora one frequently meets with 

 specimens in which the septa are not lamellar, especially in 

 species in which they are never well-developed, and on the other 

 hand many species of Montipora have well-developed lamellar 

 septa. Duncan noted in an earlier paper, which will shortly be 

 referred to, that in the genus Madrepora the septa are first 

 recognizable (in bud-corallites) as longitudinal series of spinose 

 trabeculae projecting inwards from the thin porous wall ; and I 

 find that in some species the lower part of the septa never passes 

 beyond this stage. The condition of the septa thus furnishes 

 evidence for the union of Madrepora and Montipora into one 

 family rather than for their separation. On looking elsewhere 

 for the family characters, one cannot fail to notice the close 

 general resemblance between Madrepora, Anacropora, and Mon- 

 tipora, and their equally distinct separation from Turhinaria 

 and its allies on the one hand and Porites and its allies 

 on the other. Indeed the separation into three groups is so 

 marked that, excepting in the trabeculate origin of the septa, 

 one fails to find any close relationship between the genera 

 Montipora and Anacropora and the remaining Poritinse in sensu 

 Duncan. 



Duncan in 1884 gave an account of the structure of the coral- 

 lum in certain species of Madrepora. He pointed out that in 

 young corallites the wall has only one layer of mural tissue, 

 Avhich is costulate, finely serrate or echinulate exteriorly, ac- 

 cording to the species. An increase in mural tissue takes place 

 by the formation of a new layer around the costulate or echin- 

 ulate surface, in such a way that the costulse or spinules act as 

 props for the newly-formed layer, and the space between adjoining- 

 props is converted into a more or less completely closed chamber. 



