THE SESSILE BARNACLES. 



Occludent margin 



Pit for adductor muscle X~ 



Adductor ridge 



Tergal margin 

 -. Articular ridge 



into recipient grooves of similar shape on the opposed sutural edges 

 outside of the aUe. In a few forms the radii are permeated with 

 pores which open in the intervals between the septa, and have a 

 direction at right angles with the pores of the parietes. (See plate 10, 

 fig. 2, where the denticulate septa and the ends of the parietal pores 

 are shown.) 



The basis, when calcareous, may be either solid or provided with 

 radiating pores, or it may have radiating ridges on the upper surface. 



Homologies of the 

 plates of the watt. 

 In the absence of 

 paleontologic evi- 

 dence, we may 

 assume that all 

 acorn barnacles 

 descended from a 

 primitive stock 

 having eight mural 

 compartments. 

 The most general- 

 ized genus now ex- 

 isting, Catophrag- 

 mus, has eight main compartments, with numerous smaller ones 

 outside, the latter representing the upper scales of the peduncle of 

 pedunculate ancestral forms. 



Apex 



Articular 

 ridge 



JV- -\- . Articular furrow 



Pit for lateral df- 

 pressor muscle 



Basal margin 



FIG. 3. INTERNAL VIEW OF SCUTUM. 



Scutal 

 margin 



Crests for 

 depressor 

 muscles 



Basal 

 margin 



Spur 



FIG. 4. EXTERNAL AND INTERNAL VIEWS OF TERGUM. 



Darwin and others have compared this structure with that of the 

 pedunculate genus, Pollicipes ( MiteUa) . A nearer likeness, perhaps, 

 exists with ScyUselepas, in which there has been further specialization 

 of the plates. Brachylepas, though superficially like Catophragmus, 

 is evidently not in the line of descent of the Balanomorpha. On 

 account of the different system of imbrication of the plates, it does 

 not seem likely that the Balanomorpha descended from Mesozoic 

 ScyUselepas, but rather that both had a common ancestor. 

 4729 Bull. 9316 2 



