238 robert h. wolcott: 



measurements: 



Male Female 



Length of body 1.170 mm. 1.660 mm. 



Length of leg I 0.724 mm. 0.949 mm. 



Length of leg II 0.994 mm, 1.372 mm. 



Length of leg III 0.938 mm. 1.255 mm. 



Length of leg IV 1.397 mm. 1.612 mm. 



Length of palpus ... 0.331 mm. 0.372 mm. 



Length of genital area 0.255 mm. 



Width of genital area 321mm. 



Types in the author's collection; co-types have been deposited in the 

 collection of the Zoological Laboratory, University of Nebraska, in the 

 Museum of Comparative Zoology of Harvard University, in the United 

 States National Museum, and in the Kgl. Museum fur Naturkunde in 

 Berlin. 



Atax tumidm was taken at Lake St. Clair in the summer of 

 1893, but only once, in Margaritana deltoides. At Ann 

 Arbor, Mich., three specimens were found in the collection of 

 the Zoological Department of the University of Michigan, 

 from the Huron River, but no record had been made of the 

 species of mussel. Two were collected at Lansing, Mich., 

 but in what species of Unionidae is uncertain. 



It was only collected in North Michigan in Intermediate 

 Lake at Ellsworth, August 9, 1894, from Anodonta fragilis 

 and A. edentula — 7 specimens being secured. 



At Grand Rapids, Mich., it has been collected in Anodonta 

 subcylindracea and Undo tindidatus (once) from Grand River; 

 in Anodonta ovatu (?) from Plaster Creek; and in Anodonta 

 subcylindracea from the mill-pond at Mill Creek, near Grand 

 Rapids. 



It thus has been taken only in Michigan and there but 

 rarely, yet seems generally distributed through the state; it 

 also seems to be restricted to Margaritana and Anodonta, and 

 has proven most abundant in A. suocylindracea. 



13. ATAX (NAJADICOLA) INGENS (KOENIKE) 



Atax ingens Koenike, 95b; 219, PI. Ill, f. 65 to 67. 



Najadicola ingens Piersig, 97; 60. 



A. {N.) ingens is the lai-gest of all our parasitic mites, especially if we 

 consider the size of the gravid females, one of which has been collected 

 measuring 6.0 mm., though the average length of the females is from 2.25 

 to 2.50 mm., and the males from 1.50 to 1.65 mm. The body is broadly 

 oval, evenly rounded at both ends, and much arched, highest posteriorly 



