NORTH AMERICAN SPECIES OF THE GENUS ATAX 225 



margins, those on only the basal segment of I, but gradually including 

 more and more on each leg, till on IV are serrate spines on all seg- 

 ments, and on the distal is a row of very prominent, flattened, blade-like, 

 serrate spines. On IV 4 are also three heavy club-shaped spines. In the 

 male serrate spines are present, but less numerous and not so prominent, 

 while the sex is characterized by not only the shortening of the last pair of 

 legs but also by a thickening of the same making them stouter than the 

 two preceding pairs and about as stout as the first. The claws are sharply 

 curved, have an accessory tip at two-thirds the distance from the base, 

 and end in a very fine point. 



Genital area broader than long, each lateral plate divided in the 

 female into two, an anterior pouch-shaped plate with its inner end pro- 

 duced into along neck, curved backward and bearing on its tip a flattened 

 spine, and a posterior nearly circular plate. Three specimens examined 

 possessed the following number of acetabula: 15 and 31 on one side, 12 

 and 25 on the other; 11 and 21 and 12 and 19; 8 and 12 and 6 and 17. The 

 male has but a single plate on either side with a total of 19 or 20 acetabula 

 on each side. 



MEASUREMENTS: 



Male. Female. 



Length of body 0.868 mm. 1.170 mm. 



Length of leg 1 0.668 mm. 0.816 mm. 



Length of leg II 0.820 mm. 1.020 mm. 



Length of leg III 0.870 mm. 1.070 mm. 



Length of leg IV 0.959 mm. 1 .214 mm. 



Length of palpus 0.510 mm. 



Color of legs, a bright blue-green, and body tinged with bluish-green. 

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

 Zoological Department, University of Nebraska, in the Museum of Com- 

 parative Zoology of Harvard University, in the United States National 

 Museum, and in the Kgl. Museum fur Naturkunde in Berlin. 



Of A. serratus, specimens have been collected at L. St. 

 Clair, probably from TJnio luteolus. At Grand Rapids, Mich. , 

 specimens have been obtained from TJnio cocci?ieus, TJ. imdiu- 

 latus and TJ. alatus from Grand River, and from Margaritana 

 ddtoides from Rogue River, Kent County, Mich. In material 

 from TJnio occidens from Cayuga, Cheektowaga, Erie County, 

 N. y., one specimen was found, while specimens have been 

 secured amongst a number of mites from " TJnio spp. " col- 

 lected in the Cedar River, Mt. Vernon, Iowa. It is thus a 

 widely distributed species, though one of the rarer ones. Owing 

 to an accidental interchange of the marks " % " and " ° " upon 

 the labels of two slides and the distraction of numerous inter- 

 ruptions when the preliminary description of this species was 

 15 



