Iviii THE VOYAGE OF H.M.S. CHALLENGER. 



1. Etmicca, Lamouroux, Hist, des Polyp, flexibles, p. 431, 1816; Milne-Edwards, 



Hist. Nat. des Coralliaires, t. i. p. 146. 



This geuus is characterised by the polyps having prominent calyces, which usually 

 have a projecting lip-iike portion. The thick coenenchyma has an outer cortical layer of 

 foliaceous or spinose clubs, and an inner layer of spindles. 



2. Plexaura, Lamouroux, Hist, des Polyp, flexibles, p. 424, 1816; Milne-Edwards, 



Hist. Nat. des Coralliaires, t. i. p. 152, 1857; Kollikev, pro parte, Icones 

 histiologicse, pt. ii. p. 138, 1865. 



The genus has a horny axis, but the polyps have no projecting calyces. The 

 coenenchyma includes an outer cortical layer of club-shaped or spinose spicules, and an 

 inner layer of spindles. 



3. Plexauroides, n. gen. 



The colony is branched, mostly in the one plane. The axis is horny, with a central 

 calcareous portion. Coenenchyma thin, outer layer of spicules foliaceous clubs, the folia 

 of which project, forming a rough surface. There is an inner layer of irregular stellate 

 forms. The poly^js are numerous, fully retractile, the verrucse are scarcely prominent, 

 their edges are fringed with the projecting folia of the foliaceous clubs. 



4. Plexaurella, Kolliker, Icones histiologies, pt. ii. p. 138. 



This genus is distinguished from Plexaura, which it resembles in external habit, by 

 the partially intercalcified axis, and by the spicules. The latter consist of tri- and tetra- 

 radiate forms, and of simple or foliaceous clubs. The spicules show a tendency to develop 

 twin, triple or quadruple forms. The tentacles of the polyps contain spicules extending 

 into the pinnae. 



5. Pseudoplexaura, n. gen. 



The colony is but feebly branched. The axis is horny, sometimes with calcareous 

 particles interspersed. The ccenenchyma is thick, membranous, the outer layer is friable 

 with colourless spiny spindles, the inner layer is thick with coloured stellate spicules. 

 The polyps are numerous, arranged in a spiral manner. The polyps are completely 

 retractile, without spicules in the tentacles. 



