﻿On Unionicola as a Valid Generic Name. 223 



XXII. On Unionicola Hald., as a Valid Generic Name. 



By Wm. Williamson. 



(Kead 22nd March 1909; received 24th March 1909.) 



For many years Atax F. was considered to be a valid 

 generic name until in 1897 A. C. Oudemans pointed out 

 that it was only a synonym of Hydrachna as established by 

 0. F. Mliller. ISTevertheless, Atax F. retained generic rank 

 until 1905 when Wolcott rejected it and substituted 

 Unionicola Hald. for it. The facts leading up to this may 

 be briefly stated as follows : — Linnaeus apparently intended 

 Acarus aquaticus as a name for water-mites in general 

 but later it was restricted to a species now known as 

 Limnochares aquaticus (L.). This species, however, was not 

 included by 0. F. Miiller among the forty-nine species 

 described and figured by him and for which he established 

 the generic name Hydrachna. J. C. Fabricius evidently did 

 not recognise Hydrachna, as in 1793 he included among 

 the Trombidiums thirty- three of MuUer's species and also 

 the Acarus aquaticus of Linnaeus. Latreille in 1796 

 divided Miiller's Hydrachna into two genera, viz., Hydrachna, 

 with H. cruenta Mlill., as type, and Eylais, with H. extenclens 

 MiilL, as type. To these two genera was added another, 

 Limnochares, with Acarus aquaticus L., as type. In 1805 

 J. C. Fabricius removed the water-mites, with the exception 

 \ of Acarus aquaticus L., from Tromhidium and placed them 

 in his new genus Atax. In 1834 Duges restricted Atax 

 and selected as types for it the Hydrachna histrionica and 

 ; H. lutescens of Hermann. In 1842 Haldeman described 

 I nine species of water-mites, all of which, he states, " without 

 exception, are found upon, or between the branchiae, or 

 between the branchiae and the body of the Unionidae." For 

 these he proposed the generic name of Unionicola. - U. 

 oviformis Hald. appears to be the same species which Bonz 

 earlier described as Acarus ypsilophorus. 



From 1835 to 1841 C. L. Koch added several vaguely 

 described species to the genus Atax but he did not give 



