No. 6.] NEW SPECIES OF THE GENUS ATAX BRUZ. 283 



lanceolate spines with both edges sharply serrate. On the 

 fourth segment of the posterior legs are three heavy club- 

 shaped spines. In the females serrate spines are present, but 

 less numerous and not so prominent. The claws have the 

 accessory tip at two thirds the distance from the base. The 

 epimera cover only the anterior two thirds of the ventral body 

 surface, and the spaces between them are wider. The sexual 

 area is broader than long, and in both sexes there are two 

 plates on either side, in the male the number of acetabula vary- 

 ing from 12 and 25, respectively, to 15 and 31, in the female 

 from 8 and 12 to 6 and 17. 



Twenty-eight specimens from Lake St. Clair and Grand 

 Rapids, Mich., and Cheektowaga, N.Y., taken from Unio 

 occidens Lea, U. coccincus Hild., U. undulatus Barnes, and U. 

 alatus Say. 



A.fossulatits (Koenike). 1 



The male differs from the female described by Koenike in 

 the greater length of the legs in proportion to the length of 

 the body, the somewhat larger epimera and narrower spaces 

 between them, and in the fact that the acetabula are placed 

 on two plates, one on either side of the sexual opening, instead 

 of being imbedded in the surface of the body. 



V. A. stricta n. sp. 



With the preceding were collected numbers of an Atax, 

 which apparently was A.fossnlatns, differing on ordinarily close 

 observation only in the fact that the five acetabula were in 

 one line, instead of the two posterior being side by side. At 

 first looked upon as males of A. fossulatus, the contrary was 

 only perceived when careful microscopical examination of 

 mounted specimens revealed the true males of that species and 

 led to a minute examination of several specimens. The differ- 

 ences are found to be slight, but constant. The body is broader 

 and averages less in length, the legs are slighter, the distal 



1 Koenike, " Nordamerikanische Hydrachniden." Abhdlgn. d. naturwiss. Ver. 

 Bremen, Bd. xiii, Heft 2, p. 221, Taf. in, f. 68-71. 1895. 



