278 W. SAVILLE-KENT, CONTRIBUTIONS TO A 



most readily comprehended clue to the generic groups into which 

 the Hydrachnidae, by the common consent of the students of 

 this order, are naturally divided. By 0. F. Miiller, the earliest 

 biographer of this group, all its members were included in the- 

 single genus Hydrachna, and the diagnoses of the various species 

 based merely on the varying colours and broadly perceptible 

 divisions of external contour. C. L. Koch, who up to the present 

 time has figured and described the greatest number of known 

 species, while citing the distinctions of the palpi and mandibles,, 

 entirely overlooked the modifications of the genital plates and, 

 laid the greatest stress upon the number and relative positions 

 in the dorsal region of the apertures of the mucous or dermal 

 glands, his so-called stigmatal openings. As a matter of practical 

 experience it is found, however, that the characters thus sug- 

 gested by Koch are amongst the most obscure and difficult to 

 recognise, and, moreover, in but few instances conform to the- 

 formulae he has submitted. Dujardin, in his account of the four 

 or five species only examined and described by him, was the first 

 to place on record the distinctive characters afforded by the 

 genital plates, but neither to him nor to the authorities who 

 have succeeded him does it seem to have occurred that the 

 modifications of these structures afforded a sound basis for 

 generic diagnosis, Kramer, Neuman, Haller, Lebert uniting 

 under the same generic headings forms in which the greatest 

 divergence obtains with respect to the character of the genital 

 plates and many other correlative modifications. That the 

 character of these plates afforded the most readily apprehended 

 clue to the generic grouping of the Hydrachnidae was re- 

 cognised and selected for future use bv the author of this- 

 paper in connection with the series collected and preserved so. 

 far back as the year 1868, and the more extensive knowledge 

 of the group that has been more recently accumulated through 

 the investigations of the abov^e-named authorities have but 

 tended to confirm and permit the extension of this selected 

 standard. 



In the modifications of the genital plates of the Hydrachnidae,. 

 which, as before stated, we have regarded as the homologues of 

 the basal elements or epimera of a ninth undeveloped limb 

 system. Nature seems indeed to have produced a sort of heraldic 

 code for the special benefit of systematic zoologists that may be^ 



