GAMMARUS. 367 



much smaller than those of the first four. The last pair 

 of caudal appendages are double-branched ; and the 

 central tail-plate is double. 



We have here restricted the genus to Gammarus 

 proper, believing it to be more scientifically correct to 

 group those species as distinct genera which would, in a 

 less-defined description, be arranged in subdivisions as 

 have been done by other authors. 



The spines upon the posterior segments of the tail are 

 a constant feature in our British species. In some 

 Arctic forms they are considerably increased, and in 

 G. verrucosus, Brandt, they thickly stud the whole of the 

 dorsal surface of the posterior half of the animal. 



Species of this genus appear to be abundant in the 

 northern regions, where they attain their largest propor- 

 tions ; but of forty-two species (of which six are given as 

 doubtful in the " Catalogue of Amphipoda of the British 

 Museum"), only two, G, Verreauxii (Edwards) and G. ru- 

 bromaculatus (Stimpson), are recorded from the southern 

 hemisphere, and G. Redmanni (Leach) from the tropics. 



We regard this genus as the type of the Amphipoda, 

 and as the Amphipoda may in certain respects be con- 

 sidered as the types of the class, it follows that the 

 Gammari may be looked upon as typical Crustacea. 



As will be observed from the synonymy of the species 

 of this genus very great confusion has been introduced 

 into the nomenclature, especially of the more common 

 species, arising from four difierent causes : firstly, from 

 the insufiicient and inaccurate manner in which distinct 

 species have been described, sometimes arising from a 

 definite expression being misapplied to parts homologi- 

 cally distinct ; * secondly, from several species having been 



* We have frequently had to remark the repeated application of the same 

 names to expi-ess different parts. 



