STRUCTURE OF SHELL. 39 



something extraneous, like so many spines; not that I 

 mean to say that these points of shell are dead; on the 

 contrary, they are often porose and penetrated by numerous 

 threads of corium. This upper part of the shell, thus pro- 

 duced so as to form an orifice, no doubt serves to protect 

 the less strong and moveable operculum. 



Number and Arrangement of the Compartments. — I have 

 already stated that the shell, in every one of the Balanidae, 

 consists of eight, six, or four compartments, or of all fused 

 together into a single piece ; and that the compartments 

 themselves are all constructed on the three simple patterns 

 of which woodcuts (figs. 1, 2, 3) have been given. They 

 are arranged in a certain definite order. The type arrange- 

 ment is found amongst the Chthamalinge, as might have 

 been expected, inasmuch as this sub-family is so closely 

 related to the ancient genera Pollicipes and Scalpellum, 

 whence all the Thoracic Cirripedia may be said to radiate. 

 In Octomeris (fig. 4) the type-arrangement of the corn- 

 Fig. 4. Fig. 5. Fig. 6. 

 Octomeris. Chthamalus. Chama3sipho. 



a 



Fig. 8. 

 Tetraclita. 



a, Rostrum ; I, Rostro-lateral, c, Lateral, d, Carino-lateral compartment ; e, Carina. 



Horizontal sections through the Shells of the principal genera of Balanidfe, showing the arrangement 

 of the Compartments. Genera 4?, 5, and G helong to the Chthamalinse ; 7 and 8 to the Balanitis:. 



partments, eight in number, is well shown ; the rostrum 

 and carina resemble each other, and have ala3 on both 

 sides, and therefore are overlapped on both sides : the 



