52 BALAN1D.E. 



ture. The Terga differ considerably in outline in the diffe- 

 rent genera and species : their shape approaches more nearly 

 to a triangle than to any other regular form ; but there is 

 generally a projection or spur on the basal margin, on the 

 side towards the scutum. In some species of Pyrgoma, the 

 tergum is of so irregular a shape as to defy description. 

 In most cases, a longitudinal depression or furrow runs 

 down the valve, from the apex to the extremity of the 

 spur ; and it not rarely happens that the sides of this furrow 

 become folded inwards and almost closed. The spur pro- 

 bably answers to the basal point of the usually sub-rhom- 

 boidal tergum in Pollicipes and Scalpellum.* The tips of 

 the terga in some species of Balanus, &c, are specially 

 modified into sharp points or beaks (PL 2, fig. 3 b, 3 d), 

 bowed a little inwards, and projecting considerably above 

 the tips of the scuta ; this is effected by the medial, upper- 

 most part of the valve being internally thiefcened and 

 hardened, and then, by the disintegration of the two margins 

 and the external surface, the internal modified portion be- 

 comes exposed. The whole valve, also, at least in such 

 cases as in Balanus psitfacus, appears to be forced slowly 

 upwards in the articular furrow of the scutum. I am 

 assured, by a competent observer, that the beaks of the 

 terga in B. porcatus can give an object placed within the 

 orifice of the shell a sharp tap. 



The scutum and tergum, with the few exceptions above 

 stated, are articulated together at a large or open angle. 

 The articulation (see PI. 11, fig. 5 b, c, d, and fig. 6 b, c) is 

 effected by the margin of the tergum being a little inflected, 

 and lodged in a furrow in the margin of the scutum. This 



* In comparing the Tergum of one of the Balanidac with that of a typical 

 member of the Lepadidse, for instance, that of Balanus with that of Pollicipes, 

 apex corresponds with apex : the extremity of the spur in Balanus corresponds 

 witli the basal point of the whole valve in Pollicipes : the scutal margin, (which 

 in Balanus homologically extends down to the extremity of the spur), corres- 

 ponds with the scutal margin of Pollicipes : the carinal margin in Balanus 

 corresponds with the upper carinal margin in Pollicipes : the basal margin of 

 Balanus on the carinal side of the spur, corresponds with the hirer carinal 

 margin in Pollicipes : lastly, (and this is the chief difference), in Balanus there 

 is no appreciable occludent margin, the apex of the valve being brought close 

 to the upper angle of the scutal margin; in Chthamalus, however, there is 

 yet left some remnant of an occludent margin, — which margin in Pollicipes is 

 conspicuous. 



