Hydrachnellae 271 



not open by way of the anus. The anus is present, but functions en- 

 tirely as an opening for the single, dorsomedian, excretory tubule. The 

 sexes are separate; sexual dimorphism is marked in the genito-anal 

 plate and genitaha; other structures are usually (but not always) iden- 

 tical in form in both sexes. Three eye spots are apparently always 

 present, the corneas number two, four, or six or may be absent. The 

 median eye has no cornea. There are three pairs of genital suckers. 

 These mites are principally marine, but there are numerous fresh- 

 water forms ("Porohalacaridae" of Viets). They are not capable of 

 swimming and are predominantly predaceous or herbivorous, although 

 three parasitic genera are known. Newell 1947 gives keys to the sub- 

 families. 



Rhombognathinae Viets, 1927 ) 



Genera: 



1. Rhombognathides Viets, 1927 

 Type. Aletes pascens Lohmann, 1889 



2. Isobactrus Newell, 1947 



Type. Aletes setosus Lohmann, 1889 



3. Metarhombognathus Newell, 1947 



Type. Rhombognathus annatus Lohmann, 1893 



4. Rhombognathus Trouessart, 1888 

 Type. Pachygnathus notops Gosse, 1855 



Actacarinae Viets, 1939 

 Genus: 



Actacarus Schulz, 1936 



Type. Actacarus pygmaeus Schulz, 1936 



Halacarinae Viets, 1927 

 Genera: 



1. H alacar us Go?,SQ, \^55 



a. Halacarus s. str. 



Type. Halacarus ctenopus Gosse, 1855 



b. Thalassarachna Packard, 1871 

 Type. Acarus basteri Johnston, 1836 



2. Agaue Lohmann, 1889 



Type. Halacarus parvus Chilton, 1883 



3. Agauopsis Viets, 1927 



Type. Agaue brevipalpus Trouessart, 1889 



4. Co/o^oc^-ra^ Trouessart, 1889 



Type. Coloboceras longiusculus Trouessart, 1889 



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