Mr. R. T. Pocock on Pherusa fucicola, Leach. 533 



Clearly, then, in its literal sense the rule must mean that 

 the description should be so exact as to differentiate the 

 species from all others previously known and from all that 

 will be brought to light in the future. But for all practical 

 purposes this is impossible. All that those who hold to it 

 can expect is that an author should point out such characters 

 as are believed in his day to be essential. This I believe to 

 be a legitimate, nay, the only possible practical rendering of 

 the rule ; and in accordance with this interpretation of it I 

 maintain that Leach described P. fucicola. 



This last assertion, however, requires justification because 

 it is diametrically opposed to what Mr. Walker, who should 

 know far better than I, says on the subject. 



This author writes as follows (p. 421) : — " The solitary 

 species therefore on which Leach founded his genus 

 Pherusa disagrees in almost every particular both with his 

 definition of the genus and of the subdivision in which he 

 placed it ! " This statement is substantially true of the 

 description published in the ' Linnean Transactions ' for 

 1815 ; but it is not true of the original description which 

 appeared a year earlier in the appendix to the article " Crus- 

 taceology " of the Edinb. Encycl. 



If we turn to this description we find the genus and species 

 characterized (allowing for the sake of brevity that the class 

 and tribal names symbolize certain characters) as follows: — 

 It is a Gasterurous (Hedriophthal/natous), Malacostr aeons 

 Crustacean belonging to the tribe Gamoiarides and to the 

 family Gammaridae. This family and its genera are dia- 

 gnosed in the following fashion : — 



Fam. CtAMMauid^e. 



Last joint of antennae composed of several minute articulations ; upper 

 pair longest, four-jointed; under ones five-jointed. 



a. Second pair of feet larger than first, with a 



compressed hand Melita, Mcera. 



b. Four anterior feet nearly equal in size and 



form, with ovate hands Gammarus, Amphitho'e. 



c. Four anterior feet with a filiform hand Pherusa. 



To this, the first published description, we must in all 

 fairness appeal ; and this description is true of the specimens 

 of Pherusa that Leach had *. Moreover, it enabled Leach 



* I will not go so far as to assert that I should have described the 

 hands as filiform, although as compared with the hands of, e. g., Leach's 

 Mcera their shape may well he expressed by the words. It is enough 

 for all purposes that the statement is relatively true. 



Ann. & Mag. N. Hist. Ser. 6. Vol. vii. 36 



