(sect, c), balanus eburneus. 249 



denticuli are excessively minute. The sutural edges of the alae are 

 most delicately crenated ; the alse are largely added to during the 

 diametric growth of the shell, and above the level of the opercular 

 membrane. The parietal pores are square and rather large : they are 

 crossed by transverse septa almost close down to the basis : the 

 longitudinal septa have tolerably large denticuli at their bases. The 

 pores in the basis are crossed by numerous transverse septa. 

 When specimens grow in a group, the basis is sometimes irregularly 

 cup-formed. 



Mouth : labrum serrated with small teeth, decreasing in size down- 

 wards, on each side of the central notch. Mandibles with the third 

 tooth rather thick and blunt, and with the fourth and fifth knob-like. 

 Maxillce, with the inferior part projecting much beyond the rest of 

 the edge, and bearing two long single spines : between these two 

 spines and the large upper pair, there are, in a full-sized specimen, 

 about seven pairs of moderately long spines, feathered on their 

 sides. Outer maxillce thickly clothed with very fine spines, and re- 

 markably prominent. 



Cirri : first cirrus, with one ramus having twenty-six segments, and 

 longer by ten segments than the shorter ramus, which has sixteen seg- 

 ments : the shorter ramus, and both ramii of the second pair, have their 

 segments remarkably protuberant in front; the protuberance, in the 

 upper segments, equalling in length the supporting part of each 

 segment : rami of the second cirrus unequal in length by five segments. 

 Third cirrus with the segments only slightly protuberant ; rami con- 

 siderably longer than those of the second cirrus : at the dorsal base of 

 the pedicel of this third cirrus there is no tuft of fine hairs, as is common 

 in many other species. Sixthpair, with the upper segments elongated, 

 bearing from six to seven pairs of spines ; dorsal spines short, thin, 

 and few. 



Affinities : in external appearance of the shell, this species can 

 hardly be distinguished from some of the white varieties of B. amphi- 

 trite ; and there is a considerable resemblance, in some of the varieties, 

 in the opercular valves ; but the longitudinally striated scuta of B. 

 eburneus suffice to distinguish these certainly very distinct species. 

 Equally, or even more like externally, is this species to the B. Hameri, 

 so that I have received from an eminent naturalist in the United States 

 both species mingled in the same lot, all bearing the same name of 

 B. eburneus ; but when the internal structure of the shell is exa- 

 mined, the species are at once seen to be far removed from each 

 other. Still more close is the affinity of this species to B. improvisus, 

 both in internal and external characters : it agrees with this species in 

 the singular habit of being able to live in brackish water : these two 

 species are the only ones which have the labrum serrated with teeth, 

 graduated in size, on each side of the central notch. In the case of 

 young specimens of the var. assimilis of B. improvisus, an inhabitant 

 of the same seas with B. eburneus, the diagnosis is most difficult with- 

 out long practice ; for in the young of eburneus, the compartments are 

 only partially covered by yellow epidermis, and have a striped appear- 

 ance, the radii are sometimes very oblique, the scuta externally have 



