232 ROBERT h. wolcott: 



segment also tapers toward the tip but just at the end is broadly ex- 

 panded to receive the short thick bifid claw, which is much smaller, 

 broader and thicker than in A. fossulatus. The rounded margin of the 

 expanded tip of the segment projects above the claw beyond its base, and 

 bears two short flattened lanceolate spines which project still farther. 



Genital field similar in form to that of A. ypsilophorus though some- 

 what broader, being broader than long, like that of that species in posi- 

 tion, and finally also flanked in both sexes by a single plate on either side, 

 which bears in the male about 22 to 25 acetabula, in the female 26 to 30. 

 These are not all of the same size and two a little before the middle and 

 against the outer margin of the plate are decidedly larger than the rest. 

 Spines at the genital opening of the female similar to those of A. ypsilo- 

 phorus, heavy, curved and flattened. A few small and weak spines are 

 seen along the inner margin of each genital plate. 



MEASUREMENTS: 



Female Male 



Length of body 1.170 mm. 1.250 mm. 



Length of leg 1 0.918 mm. 0.872 mm. 



Length of leg II 1.413 mm. 1.346 mm. 



Length of leg III 1.454 mm. 1.382 mm. 



Length of leg IV 2.040 mm. 1.907 mm. 



Length of palpus 0.403 mm. 0.408 mm. 



Length of genital field 0.224 mm. 



Width of genital field 0.255 mm. 



In color this species is not distinguishable from A. ypsilophorus. 

 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. 



A. arcuata was first found at Charlevoix, Mich. , in Round 

 Lake and living in Anodonta x>'beylindracea, A. footiana and 

 Unio luteolus. At " 26 " Lake it was collected in Unio kiteolus 

 and at Intermediate Lake in Margaritana rugosa, A. subcylin- 

 dracea, A. footiana, A. edentnda and A. fragilis. 



At Grand Rapids, Mich., it has been taken in Marg. 

 deltoldes, M. marginata and A. subcylindracea, and at Rogue 

 River, Kent County, in A. suhcylindracea. At White Lake, 

 Muskegon County, Mich. , it has also been found common in 

 A. subcylindracea and in all the collections referred to, far 

 more specimens were taken in that than in any other mussel. 

 It has been collected in Marg. mcvrgmata from the Schuyl- 

 kill River at Phoenixville, Pa., and specimens were received 

 from Prof. Kelly. 



