[Observed].

Polyplax alaskensis was first described by H. E. Ewing (1927, Proc. Ent. Soc. Wash., 29 : 118-121) from a single male taken from a mouse, Microtus sp., in Alaska. No subsequent collecting records of this species have been found in the literature. During the summer of 1948 a large series of individuals of both sexes was secured from mice, Microtus o. operarius (Nelson), collected on the Seward Peninsula by the writer. Since the original description is brief and unfigured, I am including here a further description of the species based on the numerous specimens now at hand. Acknowledgments are made to J. C. Bequaert of the Museum of Comparative Zoology and to Floyd G. Werner of the Harvard Biological Laboratories for aid and advice. I am also very grateful to C. F. W. Muesebeck of the Bureau of Entomology and Plant Quarantine, U. S. Department of Agriculture, for corroborating the identification by comparing a specimen from my series with the type. Female (Fig. 1, A). Length 1. 2-1.4 mm. Head almost as broad as long and generally similar to that of spinulosa ; first antenna joint much longer than the others and set close to the anterior margin. Thorax dorsally similar to that of spinulosa ; ventrally, the sternal plate is longer than it is broad; the anterior lateral margins are nearly parallel ; the posterior lateral margins are concave and slope to a blunt point ; the legs, of usual form, are of increasing size posteriorly. Pleural plates : (Fig. 2, B) first pleurite elongate, faintly if at all chitinized medially and usually with three