NORTH AMERICAN SPECIES OF THE GENUS ATAX 211 



At either side of the genital area is a conical papilla, similar to that 

 found in A. crassipes but still more prominent. 



MEASUREMENTS. 



Male Female 



Length of body 0.601 mm. 0.668 mm. 



Length of leg I 0.943 mm. 1.244 mm. 



Length of leg II 1.168 mm. 1.453 mm. 



Length of leg III 0.918 mm. 1.147 mm. 



Length of leg IV 1.199 mm. 1.601 mm. 



Length of palpus 0.301 mm. 



In the examination of mussels from Grand River, at Grand 

 Island near Grand Rapids, Mich., July 9, 1896, nymphs of 

 different species of mites and also dead mites were found in 

 the mucous about the exhalent aperature of many individuals. 

 Little notice was taken of these until in TJnio ligamentinus, TJ. 

 alatus and Anodonta edentula, a few adults were found; these 

 were, however, on cursory examination, supposed to be Atax 

 crassipes. A more thorough examination later showed them 

 not to be that species and to belong probably to Atax aculeatus, 

 a determination which has since been verified. On the 5th of 

 July, 1897, six specimens were found along the edge of the 

 mantle of TJnio spatulatus from the Rogue River, Kent County, 

 Mich. During the past summer especial care was taken to ex- 

 amine all mites and nymphs occurring in the situations referred 

 to. It was found that most of them were nymphs and of these 

 the majority were A. abnormipes, a smaller number were A. 

 aculeatus, a still smaller, A. intermedius, and now and then 

 one of A. serratus or A. fossulatus, all of these species being 

 reared from such nymphs. Of all, only A. aculeattis was rep- 

 resented by adults, while this species in its adult form seemed 

 to occur nowhere but along the margin of the mantle and about 

 the exhalent and inhalent aperatures, situations which its 

 slender form and superior activity enable it to maintain. The 

 mussels from which it was obtained were U. rectus, TJ. gibbosus, 

 TJ.undulatus, TJ. ligamentinus, TJ. occidens, TJ. ventricosus, TJ. 

 spatulatus, TJ. Novi-eboraci, TJ. coccineus, all from different 

 localities along Grand River, north of the city of Grand Rapids, 

 Mich. ; ilnio pressus and Anodonta plana, from Plumb's Creek, 



