THE CIRRI P EDI A 



HI 



Operculata is apparently Catophragmus (Figs. 02, 63), where the 

 " wall " consists of eight pieces (or " compartments," as Darwin 

 termed them), the unpaired rostrum and carina, and the paired 

 lateral, rostro- lateral, and carino- lateral plates, and is further 

 surrounded by several whorls of imbricating scales diminishing 

 in size towards the periphery, and representing the armature of 

 the vanished peduncle. In the other Operculata these scales are 

 wanting, and a series can be traced in which the compartments 

 diminish successively in number by coalescence, from Octomeris, 

 which has eight plates, through Bcdanus (Fig. 57, B) with six, and 



Elminius with four, to Pyr- 

 (jonui, where all the plates 

 have coalesced and the 

 "wall" is undivided. Each 

 compartment presents three 



FIG. 6-2. 



Catophragmus polymerus. The upper 

 figure represents the entire shell viewi-d 

 from above. .S, scutum ; T, terguin, 

 separated and further enlarged. (From 

 Gruvel's .Mnnmjritiihie.) 



Diagram showing the constitu- 

 tion of the " wall " in Catophragmus. 

 The eight principal compartments 

 are surrounded by several whorls 

 of scales. (From Gruvel's Mono- 



divisions, a central paries flanked by two lateral portions known as 

 radii or alae according as they overlap or are overlapped by the 

 adjacent compartments. The exact manner in which the over- 

 lapping takes place varies in the different genera and affords a 

 basis for systematic divisions. Thus Darwin divided his Balanidae 

 into two sub-families: (1) the Chthamalinae, in which the rostrum 

 has alae on both sides, or, in other words, is overlapped by the 

 adjacent compartments; and (2) the Balaninae, in which the 

 apparent rostrum is really formed by the fusion of the rostrum 

 with the rostro-lateral compartments (Fig. 57, B, r + rl), and con- 

 sequently has radii overlapping its neighbours on both sides. By 

 some recent writers the arrangement of the plates is interpreted 

 somewhat differently, and, though its phylogenetic importance is 

 recognised, the classification adopted is made to rest upon the 

 number of compartments distinct in the adult. 



