REPORT ON CORALS — HYDROCORALLIN^E. 101 



Stenohelia appears to form a transition in some respects between Stylaster and 

 Astylus and Cryptohelia. In it, as in the latter, all the cyclo-systems are turned 

 towards one face of the flabellum, and the styles are almost or entirely wanting in 

 the dactylopores ; its deep tubular curved gastropores, with their minute deeply-seated 

 styles are, however, very peculiar, and characteristic of the genus. 



In Astylus and Cryptohelia the dactylozooids have come to place themselves 

 out of harm's way, not by retraction within their pores, but by being doubled down 

 within the gastropore, which is divided in two chambers. They are thus enabled 

 to maintain a greater length than they could were they obliged to be retracted within 

 their own pores, and they thus obtain a longer reach. 



The lid of Cryptohelia may be a further modification of the tongue-like process in the 

 gastropore of Astylus, or the reverse may be the case, the structure in Astylus being 

 a reduction of that in Cryptohelia. If the former view be correct, then the tongue-like 

 process represents the scale often present in Errina as a covering of the gastropore, and 

 the lid of Cryptohelia is a further modification of this, which is increased in dimensions 

 and altered so as to cover an entire cyclo-system. 



Conopora forms a gradation to Astylus and Cryptohelia in being devoid of styles 

 and in having two chambers to the gastropore, but differs from them in its irregular 

 branching and the absence in it of a lid or tongue-shaped process in the gastropore. 



The phylum of the Hydrocorallina3 may, therefore, be represented as follows : — 



ArchihydrocorallinsB. 



Arcliistylasteridse. Milleporida\ 



Sporadapora. 



Errina. AUopora. Plidbofhrus. 



Distichopora. Lahiopora. Spinipora. Stylaster. 



Conopora. Astylus. Stenohelia 



I 



Cryptohelia. 



