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- 
hromaculatus (Stimpson), are recorded from tbe 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 different causes : firstly, from 
the insufficient 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 express different parts. 
