NOMENCLATURE. 3 



having very different characters, partly from undergoing a 

 varying amount of disintegration, and partly from unknown 

 innate causes, are extremely variable. Under the head of 

 Variation, in the Family Balanidse, and under the Genus 

 Balanus, I have enlarged on this subject, and have shown 

 that there is scarcely a single external character which is 

 not highly variable in most of the species. As whole 

 groups of specimens often vary in exactly the same manner, 

 it is not easy to exaggerate the difficulty of discriminating 

 species and varieties. It is absolutely necessary in most 

 cases, for mere identification, that the valves of at least 

 one specimen in a group should be disarticulated and well 

 cleaned (for which purpose caustic potash is most useful), 

 so that the internal characters may be examined. Whoever 

 attempts to make out from external characters alone, without 

 disarticulating the valves, the species, (even those inhabit- 

 ing one very confined region, for instance the shores of 

 Great Britain,) will almost certainly fall into many errors : 

 hence it is, and can thus only be accounted for, that I have 

 not seen one collection of British specimens with all the 

 species, though so few in number, rightly discriminated ; and 

 in the large majority of cases, either two or three species, 

 certainlv distinct, were confounded together, or two or three 

 varieties, as certainly not distinct, were separated from each 

 other. 



On the Names given to the different parts of Cirripedes. 



In my former volume I have .stated that I found it 

 indispensable, impart owing to the extreme confusion of 

 the nomenclature previously used, to attach new names to 

 several of the external parts of Cirripedes. Almost all these 

 names are applicable to the Balanidse, or sessile Cirripedes, 

 and to the Verrucidse ; but a few additional names are 

 requisite, which, together with the old flames, will, I hope, 

 be rendered clear by the accompanying woodcuts. In 

 sessile Cirripedes, the whole of that which is externally 

 visible, may for convenience sake be divided into the 



