THE SESSILE BARNACLES. 



into recipient gi'ooves of similar shape on the opposed sutural edges 

 outside of the ala?. In a few forms the radii are pcM-meated with 

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

 (hrection at right angles with the pores of the parietos. (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 wall. — 

 In the absence of 

 paleontologic evi- 

 dence, we may 

 assmne that all 

 acorn b arn acles 

 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 pedmicle of 

 pedunculate ancestral forms. 



Apex 



Occludent mnrgla 



Pit for adductor musde 



Adductor ridge 



-- Tergal marijin 

 .. Articular rid^o 



- Articular furrov 



Pit for lateral dp. 

 pressor muscle 



Basal margin 

 Fig. 3.— Internal view of scutum. 



Carinal 

 margin 



Basal 

 margin 



Articular 

 furrow 



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 Bala/nomorpha descended from Mesozoic 

 ScyUselepas, but rather that both had a common ancestor. 

 4729°— Bull. 93—16 2 



