GENUS B ALAN US. 193 



from being either membranous or thin, does not become 

 cup-shaped ; whereas this structure is conspicuous in B. 

 psittacus and lavis, the two commonest species in southern 

 South America. 



Fossil Species. — Having already given, under the Family, 

 some account of the geological history of sessile cirripedes, 

 short as it is, I here only allude to the subject in order to 

 state my conviction that species cannot be satisfactorily 

 distinguished in a fossil state, and rarely in a recent state, 

 without an examination of the opercular valves. Nothing, 

 indeed, could have been easier than to have affixed names 

 to many groups of specimens, having different aspects, but 

 to feel sure that these were really distinct species requires 

 better evidence than can be afforded by the shell, without 

 the operculum. No doubt, in some of the smaller sections 

 of the genus — for instance, in that characterised by a mem- 

 branous basis — it would have been possible to have distin- 

 guished some or several fossil species ; but such have not 

 as yet been found. When the specimens are much fossilised, 

 it is, indeed, difficult to make out the primary points of 

 structure — namely, whether the parietes, radii, and basis 

 are porose : to do this it is sometimes necessary to rub 

 down, polish, and carefully examine, a transverse section of 

 a piece of the shell. 



Sections of the Genus. 



[a] _ 

 Parietes, and basis, and radii permeated by pores. 



[B] ' 



Parietes and basis sometimes permeated by pores, some- 

 times not ; radii not permeated by pores ; shell elongated in 

 its rostrO'Carinal axis ; basis boat-shaped, attached to Gor- 

 yonice and Milleporce. 



M 



Parietes and basis permeated by pores ; radii not per- 

 meated by pores, 



13 



