(sect, c), balat<us concavus. 237 



With respect to the fossil specimens* from the above- 

 stated several distant localities, I consider them as cer- 

 tainly belonging to one species, though varying con- 

 siderably in several points of structure. When compared 

 with the recent specimens, they differ from them in often 

 attaining a considerably larger size ; in the parietes being 

 often, but not always, longitudinally ribbed ; and in the 

 radii often having more oblique summits. On the other 

 hand, considering the many points of identity between the 

 fossil and the recent specimen, I have concluded, without 

 much doubt, that they ought all to be classed together. 

 I may remark that, in the Coralline Crag specimens, the 

 spur of the tergum (PL 4, fig. 4 d,) is unusually long and 

 narrow ; it is broader and shorter in the Italian specimens 

 (4 e), and variable in this respect, in the United States 

 specimens ; the scnta of the Lisbon specimens are remark- 

 able for the greater prominence of the adductor ridge, and 

 for the depth of the lateral depressor cavity, Some of the 

 specimens from all the several localities are identical with 

 the recent ones from the coast of Peru. The walls of the 

 shell in the Coralline Crag specimens, are generally ribbed 

 longitudinally. I have entered into the above particulars, 

 on account of, in the first place, its offering an excel- 

 lent example how hopeless it is in most cases to make 

 out the species of this difficult genus without a large 

 series of specimens ; secondly, as showing how the cha- 

 racters alter with age ; and thirdly, as a good instance of 

 the amount of variation which seems especially to occur in 

 most of the species which have very extensive ranges. 



Some of the pink-striped Panama varieties, though having 

 a somewhat different aspect, can be distinguished from 

 certain varieties of B. amphitrite only by their scuta being 

 longitudinally striated, — a character in this species variable 

 in degree, and in most cases of very little value. Some of 

 the other recent varieties are sufficiently distinct from B. am- 

 pliitrite; and the great fossil Coralline Crag specimens, which 

 stand at the opposite end of the series of varieties, with their 

 ribbed walls, very oblique radii, and coarsely striated scuta, 

 are extremely unlike B. amphitrite. With respect to the no- 



* These will be fully illustrated in the monograph on the Fossil Balanida?., to 

 be published by the Palaeontographical Society. 



