MATTHAI— REGENT COLONIAL ASTR^ID^ 37 



described in this paper, mention is made of the histological condition of the polyps 

 sectioned. 



Key to Genera based on Polyps. 



I. Two directive couples of mesenteries present; mesenterial couples with hexameral 

 cyclical arrangement ... ... ... .. ... ... Group I. 



A. Six couples of mesenteries meeting stomodseum ; mesenterial mesogleea not 

 much thickened. 



a. Stomodaeum well formed, entocoelic pleats of mesenteries undivided ; 

 mesenterial endoderm usually swollen. 



i. Two cycles of mesenteries, each of six couples; nematocysts (III) rarely 

 present ... .. ... ... ... ... ... Cyphastrea. 



ii. Three cycles of mesenteries, the first two of six couples each and the 

 last of twelve couples ; nematocysts (III) numerous but narrow, and without any central 

 core ... ... ... ... ... ... •■■ Echinopora. 



h. Stomodaeum not well formed, often much compressed laterally and dis- 

 torted ; entocoelic pleats of mesenteries sometimes or frequently sub-divided ; mesenterial 

 endoderm thin. 



i. " Ovoid bodies " present ; numerous sub-terminal tentacular bat- 

 teries ... ... ... ... ... ... ... Galaxea. 



ii. " Ovoid bodies " absent ; up to five sub-terminal tentacular bat- 

 teries ... ... ... ... ... ... ... Leptastrea. 



B. Twelve couples of mesenteries meeting stomodseum ; mesenterial mesoglsea 

 much thickened ... ... ... ... ... ... Diploastrea. 



II. Directive couples of mesenteries absent ; mesenterial couples without any 

 cyclical arrangement... ... ... ... ... ... Group II. 



A. Number of principal couples of mesenteries indefinite ; subsidiary couples less than 

 thrice the number of principal couples ; nematocysts II and III typical Favia. 



B. Not more than six principal couples of mesenteries ; subsidiary couples 

 more than thrice the number of principal couples ; nematocysts II and III narrower, in II, 

 axis shorter ... ... ... ... ... ... ... Goniastrea. 



Key to Genera based on Corallum. 



I. Corallites formed by extra-calicinal budding ; septa with or without cyclical 

 arrangement, number meeting columella more or less definite ... Group I. 



A. Corallite-walls imperforate. 



a. Perithecal spines often present. 



i. Corallites small and vertical ; costal and perithecal spines when present 

 not high, their bases never fused ; not more than twelve septa {i.e. no tertiaries) meeting 

 columella ... ... ... ... .. ... ... Cyphastrea. 



