GENUS BALANUS. 1 7 { .) 



degree in the different species ; its lower end is sometimes 

 (as in B. lavis, PL 4, fig. 2 c) produced downwards as a 

 small, sharp, free style ; there is always an articular furrow 

 receiving the inflected margin of the tergum. There is 

 always an impression left by the attachment of the adductor 

 scutorum muscle ; and often its lower side is bounded more 

 or less closely by a sharp adductor ridge, running some way 

 down the valve ; this ridge is occasionally almost confluent, 

 in its upper part, with the articular ridge, and in this case 

 sometimes it forms, together with the inflected tergal margin, 

 a large tubular cavity, running up, as in B. psittacus (PI. 2, 

 fig. 3 c), almost to the apex of the valve. Almost invariably 

 there is a slight pit or depression for the lateral depressor 

 muscle ; sometimes within the depression there is a little 

 ridge, as in B. perforatus and nubilus (PL 4, fig. 3 a, and 

 PL 6, fig. 2 a); and in the case of B. vestitus,Jlosculiis, and 

 imperator (PL 8, figs. 3 a, 4 a), there are regular crests for 

 this same purpose. The rostral depressor muscle is usually 

 attached in a small pit formed by the folding over of the lower 

 part of the occludent margin : in B. imperator (PL 8, fig. 

 4 a) there are regular crests for its attachment, and traces 

 of them may be discovered in B. vestitus. 



Tergum. — This valve is more nearly triangular than any 

 other shape, with the spur more or less prominent. The 

 apex generally projects a little above the level of the scutum ; 

 in some species it consists of a triangular and solid, in others 

 (PL 2, fig. 3 b) of an almost cylindrical, extremely sharp, 

 inwardly curved, and very prominent beak. This beak is 

 generally purple ; it is sometimes hollow, and occupied by 

 a thread of corium. Its formation, and the apparent sliding 

 up of the whole tergum, so as to project above the scutum, 

 has been described under the family. From an account 

 given to me by a person who kept B. porcatus alive, the 

 beaks appear to be used, when the operculum is touched, 

 as an organ of defence, — the animal striking with them. 

 The tergal margin is more or less inflected ; and the carinal 

 margin is convex in different degrees, and, in some species, 

 is added to by upturned zones of growth. The basal 

 margin either forms a nearly straight line on opposite sides 

 of the spur, or more commonly slopes towards it in various 



