40 BALANIDiE. 



rostro-lateral compartments have radii on both sides, and 

 therefore overlap the adjoining compartments on both sides ; 

 the lateral and carino-lateral compartments have radii on 

 their carinal, and alge on their rostral sides ; and therefore 

 overlap on one side, and are overlapped on the other side. 

 Now the shell of every other sessile Cirripede differs, I 

 believe, from that of Octomeris, only in the fusion together 

 or abortion of some of the eight normal compartments : 

 in one genus, however, Catophragmus, outer whorls of 

 small compartments, arranged like the lower valves in the 

 capitulum of Pollicipes, are superadded. The genus 

 Chthamalus (fig. 5) differs from Octomeris only in the 

 carino-lateral compartments being aborted, (as will pre- 

 sently be discussed), and hence has six compartments. 

 Chamaesipho (fig. 6) differs from Chthamalus only in the 

 rostro-lateral and lateral compartments being fused to- 

 gether; and hence has only four compartments. In 

 Balanus (fig. 7) and the whole sub-family of the Balaninae, 

 the rostrum is compounded of the true rostrum, as seen in 

 the type Octomeris, and of the two rostro-lateral compart- 

 ments ; hence this compounded rostrum has radii instead 

 of alse on both sides, and there are only six compartments. 

 Tetraclita (fig. 8) and Elminius differ from Balanus only 

 in having the carino-lateral compartments absent, and pro- 

 bably aborted ; hence there are only four compartments. 

 Lastly, in Pyrgoma, all the compartments are blended 

 together into a single piece. 



In Pollicipes, the old type-form of the whole order, and 

 in Scalpellum, we have four valves, (answering to the oper- 

 culum), surrounding the aperture leading into the sack, and 

 the valves below are arranged in successive whorls, with a 

 strong tendency to alternation. For, the rostrum alternates 

 with, that is faces the interval between, the two scuta ; 

 the carina alternates with the two terga ; and the upper 

 Lateral valves alternate with the scutum and tergum on 

 each side. These four valves, namely, the carina and 

 rostrum, which resemble each other in structure, and the 

 pair of upper latera, which are larger than the other lateral 

 valves, together form the uppermost whorl, or that beneath 

 the scuta and terga. In the next whorl we have the 



