36 



BALANID.E. 



growth on both sides is plain. The modified sides (rr) for 

 convenience sake, have been called the radii; they invariably 

 overlap the adjoining compartments. The middle part 



Fig. 1. 



Fig. 2. 



Fig. 3. 



p 



p 



p,p, Parietes; r, r, Radii; a, a, Alee. 



Fig. 1, Rostrum with two radii, serving in the Chthamalinse for rostro-lateral compartments. 

 Fig. 2, always serving for lateral and carino-lateral compartments. 

 Fig. 3, Carina, serving in the Chthamalinse, also, as a rostrum. 



ip), has been called the wall, or parietal portion : in the 

 specimen figured, the walls and radii are distinctly separated, 

 but in some cases, especially amongst the Chthamalinse, the 

 lines of growth are absolutely continuous from one to the 

 other. In fig. 2 of a Lateral compartment of the same 

 Balanus, we have the same essential structure; but the left 

 side (a) is more protuberant, and is hollowed out in its 

 lower half; it is, also, more distinctly separated from 

 the parietal portion : this side has for convenience been 

 called the a/a ; it is invariably overlapped by the adjoining 

 compartment : in some few cases, as in Pachylasma, the 

 ala is not hollowed out in its lower part, and from being 

 added to in a straight line along its whole edge, with the 

 lines of growth continuous with those on the wall, it differs 

 hardly at all in appearance from a radius. Lastly, in 

 fig. 3 of the carina, or compartment facing the rostrum, we 

 have alas (aa) on both sides ; these being, as in all cases, 

 overlapped by the adjoining compartments. 



Now, the compartments in the shell of every sessile 

 Cirripede, are without exception constructed on the above 

 three simple patterns. In number, they are 8, 6 or 4, or all 

 confluent together. 



Considering this simplicity in growth and form of the 

 separated compartments, it seems at first surprising that 



