18.2 BALANIDJS. 



effected by the growth of the radii and alae, and chiefly by 

 that of the former. The statural and lateral edges of both 

 radii and alae are added to, either quite up to their summits, 

 or only low down, and during the continued growth of the 

 shell, lower and lower down ; in accordance with this differ- 

 ence in growth, the summits of the radii and alae become 

 either very oblique, or they extend parallel to the basis, 

 that is, from tip to tip of the adjoining compartments. When 

 the radii and alae are added to, as is most usual, above the 

 level of the opercular membrane, and therefore above the 

 sack, ribbons of corium run up the sutures from the sack, 

 higher or lower, according to the height to which, in the 

 different species, the edges of the radii and alae continue to 

 be added to. The obliquity of the summits of the radii and 

 alae varies, in some cases, in the same species. It often 

 happens that when the summits of the radii are very oblique, 

 the summits of the alae are but little so ; and the converse ; 

 both, however, are often either equally oblique, or both 

 have square summits. The sheath extends either one third 

 or more than half down the shell; its basal margin often 

 (PI. 25, fig. 1, k') freely depends or overhangs the inner 

 lamina of the walls. 



Basis. — In typical species the basis is calcareous, and con- 

 sists of an upper and lower lamina, separated by radiating 

 septa, forming pores. In the same manner as the septa of 

 the parietes sometimes, though rarely, become irregularly 

 divided near the outer lamina, forming outer pores, so it is, 

 but in a much more marked degree, with the basis. The 

 basis in such cases becomes extremely thick, and consists of 

 an upper, thin lamina, with the regular radiating septa 

 and pores, and of an underlying, thick, cancellated mass, 

 which seems wholly to result from the dividing and sub- 

 dividing of the septa. The basal radiating pores, like the 

 parietal pores, are closed at intervals by calcareous trans- 

 verse septa. The basal points of the parietal septa enter 

 the orifices of the basal pores, and the threads of corium 

 pass into the latter, between the denticuli of the parietal 

 septa. In some species, as in B. crenatus and Hameri, 

 the basis is perfectly solid, the upper lamina being ab- 

 sent, just as in some species, the internal lamina of the 



