180 balanidjE. 



manners. The spur, or basal projection, is rarely placed 

 in the middle of the basal margin, generally near, some- 

 times close to the basi-scntal angle ; it varies much in 

 length and breadth, and is sometimes even half the width 

 of the valve. The surface of the valve is almost always 

 more or less depressed, sometimes so much as to form 

 a deepish furrow, the " longitudinal furrow," which ex- 

 tends from the apex to the extremity of the spur. When 

 the furrow is deep, its sides, as the specimen grows old, 

 almost always become folded inwards, so as to touch, and 

 then the furrow becomes converted into a closed fissure : in 

 this latter case the folded sides generally form a central 

 crest on the spur. Internally, in the middle of the upper 

 part of the valve, the articular ridge is more or less promi- 

 nent, forming the carinal margin of the articular furrow, in 

 which the articular ridge of the scutum is lodged ; occa- 

 sionally, however, this articular ridge can hardly be said to 

 exist. In most species the tergal depressor muscle is 

 attached to sharp crests on the basi- carinal corner of the 

 valve, but these are almost obliterated in other species. 



Compartments. — The external appearance of the shell has 

 already been described. In the most typical species, the 

 parietes consist of an outer and inner lamina, separated by 

 strong longitudinal septa ; these septa are denticulated on 

 both sides at their bases, but only close to the inner lamina ; 

 in fact the inner lamina is apparently formed by the union, 

 thickening, and production, of some of the denticuli. As 

 it is not the innermost of the denticuli on the basal 

 edges of the longitudinal septa, which thus become united 

 into a solid layer, the longitudinal septa form slightly pro- 

 jecting, longitudinal ribs on the inner lamina. These in- 

 ternal ribs are longitudinally striated ; in old specimens 

 they often become obliterated, especially in the upper part 

 of the shell. The parietal tubes or pores (occupied by 

 threads of corium) are generally square and large ; but in 

 B. Ajacc they are very small, and in B. glandxda often ex- 

 tremely minute. In the upper part of the shell, and some- 

 times low down, they are generally crossed by thin, transverse, 

 calcareous septa: in some species, as in B. perforatus, and 

 in some varieties of B. ampkitrite, the upper ends of the 



