MARINE POLYZOA. 47 



Escharidae (part.), Johnst. Brit. Zooph. ed. I. 248, ed. 2. 263 ; 



Gray, Syn. Brit. Mus. 1842, 135. 

 Polypiers a reseau, Lamk. 2 ed. ii. 210 (part.). 

 Flustrees, Lamx. Exp. Meth. 2 (part.). 



The principal distinctive character between the Flustradae, as 

 here intended, and the very closely allied group of the Mem- 

 braniporidae, consists in the more or less erect and free con- 

 dition of the former. Since however, on the one hand, a Flustra, 

 as is often the case in Flustra foliacea, may be decurrent at 

 the base and spread to a considerable extent in the adnate form ; 

 and on the other, a Membranipora, as in the case of the so- 

 called Flustra membranacea (Membranipora fiustroides, mihi), is 

 occasionally, as when spreading over Flustra foliacea for instance, 

 nearly free, it would seem that this distinction is hardly suffi- 

 cient to allow of their being regarded as belonging to two distinct 

 families : the relationship, in other words, between the Flustradse, 

 as here constituted, and the genus Membranipora, is more that of 

 family and subfamily. The latter genus might therefore almost 

 be considered as the type of a subfamily, the Membraniporana. 

 The distinction again between the Flustradae and the Escharadae 

 on the one hand, and between Membranipora and Lepralia on 

 the other, seems, in the former case at least, to be, if not 

 wholly, at least in great part, artificial. The distinction drawn 

 between the flexibility of the Flustradae and the rigidity of 

 the Escharadae may perhaps be looked upon as artificial, but 

 beyond this, the Escharadae exhibit characters, principally in con- 

 nexion with the avicularia, which would indicate a more natural 

 distinction between the two groups. The distinction between 

 Membranipora and Lepralia is in some respects greater and 

 more natural, as will be indicated in speaking of those genera. 



Synopsis of Genera. 



1. Flustra. 



2. Carbasea. 



3. Diachoris. 



1. FLUSTRA. 



Cells contiguous ; on both sides of the frond. 



Flustra, sp. Linn. 



Flustra, Lamk. Syst. 1801 ; Gray, Cat. Brit. Rad. 145. 



1. FLUSTRA FOLIACEA. PL LV. figs. 4, 5. PL LVI. fig. 5. 



Cells arched and expanded above with two marginal spines on 

 each side, contracted and truncate below. Avicularium and man- 

 dible semicircular. 



