k"7 



194 • eobert h. walcott: 



Hydrachna. In 1793, J. C. Fabricius included all these under 

 Tromhidiwn, but in 1805 he established the genus Atax which 

 was equivalent to Hydrachna of Miiller. Previously, in 1796, 

 P. A. Latreille had erected the genera Limnochares and 

 Eylais, but these were by Fabricius included under Atax. In 

 1834, Antoine Duges restricted the name Hydrachna to a few 

 species which are still so classified, re-established the genera 

 Limnochares and Eylais, and separated from Atax Fabricius, 

 which included the greater number of the species, the additional 

 genera Diplodontus and Arrenurus. In 1837 the genus of 

 Fabricius was still more sharply limited by C. L. Koch, who 

 separated several new genera; but only in 1854 and by 

 Ragnar Bruzelius was the genus Atax reduced to the limits 

 which were for forty years accepted by all students of the 

 group as its natural bounds and are by many still so regarded. 

 In 1894 Richard Piersig, on grounds considered insufficient by 

 Koenike, separated from Atax the genus Cochleophorus, and 

 in the past year, 1897, he has proposed another new genus, 

 Eiin nt i ■idnjtliorus, to include a species described by Koenike 

 from East Africa, and the genus JYajadicola to include one of 

 our American species, also described by Koenike. The char- 

 acters which separate Cochleophorus from Atax, as thus 

 limited, are sufficient, it seems to the author, to render the 

 former a valid genus; certainly the two designate clearly de- 

 fined groups of species and all described forms fall naturally 

 into one or the other of these groups. In accordance with this 

 view the species included under Cochleophorus are excluded 

 from this paper. In regard to Najadicola, however, while it 

 possesses certain characters which differ from those of other 

 species of parasitic mites, the writer has been unable to agree 

 with Piersig: in thinking these differences such as to entitle it 

 to more than sub-generic rank. 



As in the above manner limited, the characters of the genus 

 Atax are thus defined by Piersig in his "Deutschlands Hy- 

 drachniden" (97): 



Body soft, with but a slight tendency to the formation of 

 chitinous thickenings over the surface, round or oval; on the 



