KNOWLEDGE OF TPIE HYDRACHNIDAE. 279 



likened to the escutcheons and armorial bearings of the human 

 families, and whereby we may deduce the co-existence of 

 ^ill those other correlative, morphological, developmental and 

 iietiologic characters upon which, with the consent of recent 

 bioloo-ists, the generic delimitations of the Hydrachnidae may 

 be most logically based. How far this proposed key holds 

 good may be the better determined after an examination of 

 the scheme, with its accompanying illustrations, that has now 

 to be submitted. 



Classificatory System (Generic Subdivisions). 



Taking the Hydrachnidae as a family group, and examining 

 the extensive modifications presented by the genital plates, it is 

 easy to recognise that the majority of these are at the outside 

 modelled upon two or three distinct plans only, or indeed that the 

 Avhole series allow, as might have been anticipated, of the inter- 

 pretation of being the derivations of some common ancestral type. 

 In the first place, it will be found that in a considerable number 

 of types, including notably such genera as Llmnesia, Lebertia, 

 Jfideopsis, Maj'ica, Miclea and Pseudoatax, the genital plates take 

 -a symmetrically ovate form remarkably resembling the ordinary 

 stomata of the leaves of plants, and differ from each other 

 only in the number, form and disposition of their tubercular 

 ornamentations. In a second ecjually abundant series, com- 

 prising such forms as Xesaea, Hydrochoreyites, Fiona, Hygro- 

 hates and Dlplodontus, the dominant contour of the conjoined 

 genital plates is that of a heart with its apex directed forward, 

 the several genera diftering only inter se, as in the former series, 

 in the number, relative size or form, and disposition of the 

 included tubercles or acetabula. Starting with these two more 

 prominent modifications, it is easy to arrive at any of the 

 apparently anomalous types. In this manner it may be 

 supposed that the prolonged tuberculated genital areas charac- 

 teristic of the orenera Aii'henurus and A^iuranea are but 

 modifications of the tuberculate heart-shaped genital areas of 

 such a form as Xesaea or Diplodontiis ; while again, in such 

 genera as Hydryphantes and Bradyhates we have apparently 

 an intermediate condition between the stomata-like and heart- 



