i895- A PASS AGE- AT-A RMS OVER THE AMPHIPODA . 263 



attributing to one generic name privileges belonging to the date of 

 another, it is necessary to pass on to matters more essential. A genus, 

 to have any validity, must be defined. It was, therefore, with no 

 little curiosity that I turned to Professor Delia Valle's definitions to see 

 how they would justify his amalgamating procedure. Clearly, defi- 

 nitions may be constructed, like fishing nets, with meshes very large 

 or very small. They maybe extremely comprehensive or exceedingly 

 restrictive. The characters which one man would think sufficient for 

 a genus might scarcely satisfy another for a family, or, on the other 

 hand, might content his neighbour for defining a species. While we 

 know so little, and are for long likely to know so little, of phylogeny, 

 the various groups remain a question of scientific convenience. But 

 at least a definition ought to define, and it cannot do this without some 

 approach to precision. Now Professor Delia Valle's definition opens 

 in this wise: — "Body for the most part compressed (in Acanthozone 

 incisa it is depressed), segmented regularly. — Back for the most part 

 armed with spinous processes." The parenthetical exception refers to 

 the " Challenger " Chosroes incisus, one of the few Gammarids in which 

 the body is dorso-ventrally flattened, instead of being compressed 

 from side to side, as is usual, not in one genus alone, but in almost 

 every genus of the Gammaridea. Moreover, in this same species the 

 back is perfectly smooth. It would seem, therefore, a most incon- 

 venient perversity to include it in a genus named from the spinous 

 processes with which in the type the back is armed, but which are 

 here only conspicuous by their absence. The next statement in the 

 definition is that the anterior antennae have the lash very long, but 

 this again is not true of Chosroes incisus. Another " Challenger " 

 species, Stenopleura atlantica, is reduced to the ranks of Acanthozone, 

 although it also is guiltless of any dorsal armature, and although the 

 side-plates, instead of being, as Delia Valle's definition requires, 

 " of moderate size," are conspicuously small. Anyone who will be at 

 the pains to compare the detailed figures of this species with those 

 of Acanthechinus tricarinatus in the Report on the " Challenger " 

 Amphipoda will, I think, admit that, if Delia Valle has succeeded in 

 his attempt to make a happy generic combination of these two, the 

 time has indeed come when the lion may safely lie down with the 

 lamb. To be sure, they both agree with the definition in having the 

 " gnathopods subchelate." There are very few of the Gammaridea 

 in which this is not so. The gnathopods, or two foremost pairs of 

 legs, have in Stenopleura the penultimate joint or " hand " almond- 

 shaped, seated in a short, cup-like "wrist," with a long "finger" 

 curving over the margin of the hand to impinge against the "wrist," 

 thus forming a sort of claw as effective perhaps as a lobster's, but of 

 a different pattern. In all this there is nothing unusual or out of the 

 common. It might almost be taken as a typical example of the 

 structure of these limbs in the Gammaridea. Then look at the other 

 genus, in which a very short finger does little more than cap a very 



