340 (SIDNEY F. IIARMER. 



'' frontal shield " is suggested) is developed as a series of 

 marginal spines, which overarch the frontal membrane. This 

 leads to the condition found in some Escharine forms, in 

 which, as in Umbonula, the calcareous frontal shield 

 similarly overarches the frontal membrane. 



(4) Tlie frontal shield of the Microporid;i3 and Stegano- 

 porellidte is of the nature of a cryptocyst,^ — that is to say, of a 

 calcareous lamella, which grows horizontally across the bod}"- 

 cavity beneath the fi'ontal membrane. This has, perhaps, 

 led to the condition found in Lepralia, Schizoporella, 

 and other genci'a, in which the compensation-sac is developed 

 as an invagination at the base of the operculum and passes 

 to the deeper side of the frontal shield, which is on this view 

 a cryptocyst. 



(5) The epitheca, or layer of living tissue, bounded by a 

 cuticle, which covers the frontal shield, may represent the 

 entire frontal membrane (Microporidffi, etc.), or only a part 

 of that membrane (Lepralia, etc.), or it may have been 

 derived from the outer calcareous layer of the frontal bars of 

 a Cribrilina-like form (Umbonula). The epitheca is 

 frequently responsible for the addition of second ai'y calcareous 

 matter to the frontal shield, and in some cases (Cellepora, 

 etc.) for the formation of new generations of zooecia which 

 are superposed on the older ones. 



(6) The operculum, in its more primitive condition, is merelv 

 part of the frontal membrane strengthened by a semicircular 

 marginal flange. It acquires a firmer texture and a more 

 elaborate arrangement of its thickened parts as the result of 

 its articulation with calcareous portions of the zooccinm. 

 While occlusor muscles may be regarded as an essential 

 adjunct of the operculum, definite divaricators seem to have 

 been evolved as a modilication of the distal pair of parietal 

 muscles. 



(7) 'IMie consideration of the relations of the frontal surface 

 involves consider.able rearrangements in the Clieilostoniata 

 (see section on classification, p. 329). 



' Cf. Harmer (1900), p. 228. 



