STRUCTURE OF SHELL. 41 



rostro- and carino-lateral valves, alternating with those 

 above them ; and beneath them there are generally other 

 valves, which decrease in size and display the same tendency 

 to alternation. The valves here just specified, namely, the 

 rostrum, carina, and three pairs of lateral valves, in the 

 Lepadiclse, are so much larger, and are so much more 

 commonly present, than the other valves of the capitulum, 

 that to them alone I affixed special names. Now if amongst 

 sessile Cirripedes we look to that genus, viz., Catophragmus, 

 which comes in its whole structure the nearest to Pollicipes, 

 one of the Lepadidae, we find (as in fig. 4), firstly, a rostrum 

 and carina resembling each other, and a pair of lateral com- 

 partments, larger than the other lateral pairs ; these four 

 valves alternating with the opercular valves : and, secondly, 

 we find, but forming part of the same whorl, a pair of rostro- 

 lateral and a pair of carino-lateral compartments, which, just 

 as in Pollicipes, are larger than the exterior and lower valves. 

 These lower little valves, I may remark, decrease in size in 

 the successive whorls, and tend to alternate in position, just 

 as in Pollicipes. Observing these several striking points of 

 correspondence in the valves, (and indeed in the whole 

 structure), of Catophragmus and Pollicipes, one is strongly 

 inclined to suspect that in Catophragmus, and therefore in 

 Octomeris and other sessile Cirripedes, although the rostro- 

 and carino-lateral compartments appear to lie in the same 

 whorl with the rostrum, carina, and large lateral compart- 

 ments, yet that they really belong, as in Pollicipes and 

 Scalpellum, to a lower whorl. Now if a very young shell 

 of Balanus, immediately after the metamorphosis, be ex- 

 amined, the carino-lateral compartments will be found not 

 to have been developed ; they first appear after two or three 

 zones of growth have been added to the other compart- 

 ments; bearing in mind that in Pollicipes and in Cato- 

 phragmus the lower whorls are added successively during 

 growth, we find in this fact strong confirmation of the view 

 that the carino-lateral compartments normally belong to a 

 whorl beneath that including the rostrum, carina, and 

 lateral compartments. Whether the rostro-lateral, like the 

 carino-lateral compartments, are developed subsequently to 

 the others, I have had no opportunity of ascertaining, and 



