CAP1TULUM. 29 



ceed a hundred in number, they are arranged in whorls, 

 with each valve generally so placed as to cover the 

 interval between the two valves above. Of all the valves, 

 the scuta are the most persistent; then come the terga, 

 and then the carina ; the rostrum and latera occur only in 

 Scalpellum and Pollicipes, and in a rudimentary condition 

 in Lithotrya, and, perhaps, in the fossil genus Loricula. 

 The valves are formed sometimes of chitine (as in Tbla and 

 Alepas), but usually of shell, which varies from trans- 

 parency to entire opacity. The shell is generally white, 

 occasionally reddish or purple; exteriorly, the valves are 

 covered by more or less persistent, generally yellow, strong 

 membrane. The scuta and terga are always consider- 

 ably larger than the other valves : in the different genera 

 the valves differ so much in shape that little can be pre- 

 dicated of them in common ; even the direction of their 

 lines of growth differs, — thus, in Lepas and some allied 

 genera, the chief growth of the scuta and of the carina is 

 upwards, whereas in Pollicipes and Lithotrya, it is en- 

 tirely downwards ; in Oxynaspis, and some species of Scal- 

 pellum, it is both upwards and downwards. Even in the 

 same species, there is often very considerable variation in 

 the exact shape of the valves, more especially of the 

 terga. The adductor muscle is always attached to a 

 point not far from the middle of the scuta, and it gene- 

 rally has a pit for its attachment. In several genera, 

 namely, Psecilasma, Dichelaspis, Conchoderma, and 

 Alepas, the scuta show a tendency to be bilobed or 

 trilobed. The valves are placed either at some distance 

 from each other, or close together; but their growing 

 margins very rarely overlap each other, though this is 

 sometimes the case with their upper, free, tile-like apices ; 

 in a few species the scuta and terga are articulated to- 

 gether, or united by a fold. The membrane connecting 

 the valves, where they do not touch each other, is like 

 that forming the peduncle, and is sometimes brilliantly 

 coloured crimson-red ; generally, it appears blueish-gray, 

 from the corium being seen through. Small pointed 



