448 BALANIDiE. 



the difference between Chthamalus and Balanus. I was 

 first indebted to Dr. J. E. Gray for explaining to me 

 this difference ; but the only published account which I 

 have met with is in a paper by the Rev. R. T. Lowe,* in 

 which he states, on the authority of Mr. Clark of Bath, that 

 in Chthamalus the anterior compartment or rostrum has 

 alae like the posterior compartment or carina, the anterior 

 or rostro-lateral compartments being destitute of alac. These 

 characters being exactly reversed in Balanus, as I have 

 already explained (p. 176) under the sub-family of Bala- 

 ninae. 



The shell, owing apparently to its containing much animal 

 matter, is particularly subject to disintegration ; and when 

 thus much affected it is quite impossible to distinguish the 

 species by external characters. It is, in fact, best to cast 

 ou one side external appearance, though when the shells 

 happen to be well preserved, each species has its own pecu- 

 liar aspect. We have in this genus smooth and plicated, 

 cylindrical and depressed varieties of most of the species. 

 The development of the radii is very apt to vary, and even 

 the compartments often become so completely united and 

 calcified together that the sutures are almost or quite obli- 

 terated. A more serious difficulty in discriminating the 

 species, arises from the fact of the opercular valves, not only 

 varying extremely in external appearance in consequence of 

 the greater or less disintegration of their apices, and conse- 

 quent exposure of their articular ridges and furrows (com- 

 pare fig. 1 a, 1 b y 1 c, in PI. 18), but from their truly varying 

 in outline with the varying shape of the shell : this latter 

 circumstance is probably due to the opercular membrane 

 which unites the valves to the shell being very narrow, and 

 in consequence, differences in the shape of the shell affect 

 the opercular valves, in a manner and to a degree to which 

 the Balaninae are not subject (compare fig. 1 e, 1/, 1 h, in 

 PI. 18). The scuta, on the other hand, differ to an unu- 

 sually slight degree in the different species. In the com- 

 mon Chthamalus stellatus, which abounds on the southern 

 British shores, the whole external aspect of the shell is 



* 'Zoological Journal,' vol. 3, p. 76, 1828. 



