24 



MADREPORARIA. 



and modify the calicle, or was it the modification in the parent calicle which changed the shape 

 of the colony ? * 



Here we have simply to describe the facts. In the explanate colonies, the primitive type 

 of calicle either persists, see PL II. fig. I, PL VII. fig. 8, or the skeleton of the calicle gets more 

 and more flattened down by a reduction of the skeletal elements into short, thick, vertical 

 trabecule, the tops of which at the surface look like so many closely arranged granules. The 

 colonies in this case get increasingly thin, and the calicles more and more obsolete. A series 

 showing this may be seen in the following figures and in this order, PL IV. fig. 2, PL VII. 

 fig. 1, PL IV. fig. 1, PL V. fig. 4; while a different kind of flattening of the calicle, 

 viz. by the reduction of its skeletal elements into thin horizontal flakes, is also seen in another 

 explanate growth, PL V. fig. 5. 



A D 



Fig. 2. — Diagrams to illustrate the different massive growth-forms with the free edges 

 characteristic of this genus. The tabulae (/) are shown in A and B continuous 

 with the epitheca (e) under the free edges. A, general diagram ; B, a simple 

 hemispherical stock, with the calicles (c) of the living layer ; C, the stock is a 

 succession of cushion-shaped growths (pulvinate) of which the uppermost is alone 

 living, and the two lower are bounded above by tabulae. D, very rapid central 

 growth forms an expanding column with straight sides (wheatsheaf formation) ; E, 

 very rapid central growth resulting in a column, here shown forking. 



In the other line of development, towards the formation of columnar stocks, the walls of 

 the calicles in the centre of the colony grow in height, and according to variations in the speed 

 and uniformity of this central growth we get all the known modifications of the massive type. 



Fig. 2, diagram A, shows a stock apparently built up of a succession of colonies in which 



* There is evidence that slight departures from the normal growth-form will seriously change 

 the type of the calicle, see for example G. Cheat Barrier Reef 7 ; and cf. PL II. fig. 9 with PI. III. fig. 1. 



