VARIATION. 155 



as far as the shell is concerned, leads into the Balaninae, 

 and Catophragmus into the Lepadidse ; Octomeris leads 

 towards Chthamalus, and Chthamalus towards Charuaesipho. 

 Variation. — The discrimination of the species in most 

 of the genera, offers very great difficulties. I cannot 

 too strongly impress on any one intending to study this 

 class, not to trust to external characters : he must sepa- 

 rate and clean, and carefullv examine the internal struc- 

 ture and form of the compartments, and more especially of 

 the opercular valves. After considerable experience, when 

 numerous varieties of a species have been carefully examined, 

 the eye acquires a sort of instinctive knowledge, by which 

 it can recognise the species, though the character cannot be 

 defined by language ; but I have found that no amount of 

 experience with some of the commonest species, will save 

 frequent and grave errors, as long as external characters 

 alone are trusted to. Not only does every external cha- 

 racter vary greatly in most of the species, but the internal 

 parts very often vary to a surprising degree ; and to add to 

 the difficulty, groups of specimens not rarely vary in the 

 same manner. After having given up several years to the 

 study of this class, I must express my deliberate conviction 

 that it is hopeless to find in any species, which has a wide 

 range, and of which numerous specimens from different 

 districts are presented for examination, any one part or 

 organ, — which from differing in the different species is fitted 

 for offering specific characters, — absolutely invariable in form 

 or structure. I may in one respect even go further, and 

 affirm, that, if in a species, any part or organ differs remark- 

 ably from the same part in its congeners, then if many speci- 

 mens are examined, especially when collected from different 

 districts, such part or organ will be found eminently variable. 

 1 may instance the antenniformed third pair of cirri in 

 Chthamalus antenna tus, the teeth on the posterior cirri in 

 Acasta sulcata t the terga in Pyrgoma dentatum, the adductor 

 ridge of the scuta in Pyrgoma cancellatum and in Creusia, 

 and other such cases : hence it will not do to found a species 

 on a slight, or sometimes even on a considerable difference, 

 in any single point or organ. On the other hand, I am far 

 from asserting, that if only half-a-dozen specimens of some 



