ME. H. M. BKRJfABD ON RECEXT PORITIDJ;. 139 



edges, already mentioned, in whicli lamellate costse ran out to 

 the rim of the epitheca, finally convinced me that the so-called 

 " trabecular " septa are merely perforate lamellate septa. 



We haye thus reached an important stage in our enquiry as 

 to the position of the Poritidse : their so-called "trabecular" 

 structure belongs to the terminology of the past. Their thecse 

 were originally built up of lamellate plates like those of the 

 Madreporids, and the perforation of these plates has to be con- 

 sidered as a secondary characteristic. To this difference between 

 the Madreporid and the Poritid septum we shall return when we 

 have discussed the next most striking contrast between the two 

 families, which may be stated as follows : — 



In the Madreporidas, except in Mo)itipora with its immense 

 development of the coenenchyma, the thecae are tall and conical. 

 In the Poritidae, on the contrary, the thecae are low and shallow. 

 The septa in the latter are therefore not only perforated, i. e. 

 poor in quality, but also poor in quantity, that is in size. 



This contrast is shown diagrammatically in fig. 1, p. 135, ia 

 which C and D are intended to represent individual calicles 

 (ideal parent calicles) of a Madreporid and of a Porites respec- 

 tively. In the former, the septa rise above the flattened epitheca 

 to form a new theca, being mutually supported by synapticulas 

 which would project from the plane of the figure in the dotted 

 areas. In the latter we have the low basal skeleton of Porites ; 

 the septa with their synapticulae being together reduced to a 

 reticulum. Can any explanation be given of these differences? 

 I think so. The diagrams of theimselves seem to suggest that 

 the conditions found in Porites are due to arrested development. 

 The suggestion is therefore made that these swarms of minute 

 polyps, which are so ubiquitous and appear in such vast numbers 

 that they are reckoned among the principal builders of the coral- 

 reefs, may be regarded as Madreporids arrested at an early stage 

 in their development. This is, in fact, the position I have found 

 myself compelled to assign to the Poritidse in the Catalogue of 

 the British Museum Madreporaria *. 



* Vol. T. (by the late Qeorge Brook) deals with Madrepora. Vols. II. & III. 

 contain the Madreporid genera Turhinaria, AstrcBopora, Montipora, and Ana- 

 cropora. Vol. IV., which is nearing completion, contains the two Poi-itid 

 genera Porites and Goniopora, somewhat extended (see below, pp. 143-148). 



lilNN. JOUEN. — ZOOLOGY, VOL. XXVII. 11 



