﻿part 4] PHILLIPS ASTHMA HENNAHT, AND ORION ASTHMA. 283 



Fig. 1. — Longitudinal section t of Orionastraea placenta (McCoy), 

 liolotype of Sarcinula placenta, showing the corallite in its 

 earliest stage, x 5 diameters. 



W. Tarns photo. 



(ii) The development of dissepimental tissue can be shown to be 



progressively greater, as we follow the passage of dendroid 

 and fasciculate into basaltiform, and these into astrseiform 

 colonies. 



The case is not merely one of phylogenetic change, but may 



be observed to take place wherever 

 Fig. 2. — Group of cur all it es two or more corallites of a den- 

 qf Lithostrotion martini, droid-fasciculate colony approach 

 X 2 diameters. each other to the extent of con- 



tiguity. This is illustrated by the 

 accompanying figure of a group of 

 corallites of Lithostrotion martini 

 Edwards & Haime. 



The phenomenon may be accoun- 

 ted for, by supposing that two 

 neighbouring polyps mutually ex- 

 tend one. towards the other until 

 they are in contact. This results 

 in a stretching of the area of dis- 

 sepiment-secreting tissue where the 

 two touch, and in a consequent 

 widening of the dissepimental area of the skeleton when secreted. 



(iii) The external epitheca, which stretches round the corallum as 



a continuous surface, usually displays pronounced rugosity. 



It differs neither in its origin nor in character from the 

 epitheca that clothes a simple coral or an individual corallite ; 

 yet its presence is often emphasized in descriptions of corals 

 (especially if the corallum in question is discoid in form), and 



