10 PROCEEDINGS OF THE NATIONAL MUSEUM vol.82 



a long bladelike structure, grooved and ending in a spurlike process ; 

 fixed chela absent. Length, 0.45 mm ; width, 0.3 mm. 



Type host. — Microtus calif ornicus. 



Type locality. — ^Los Angeles, Calif. 



Type s^^c?e.— U.S.N.M. no. 1075. 



Remarks. — Description based on three females and two males from 

 type host, a meadow mouse, at the type locality, April 1926 (U.S. 

 Public Health Service), and three females from a young Microtus 

 sp., Flat, Alaska, March 29, 1925, by A. H. Twitchell. 



Genus CERATONYSSUS Ewing, 1923 



Body much longer than broad and sparsely clothed with short 

 setae. Dorsal j^late of female divided, posterior part large, not cir- 

 cular. Chelicerae shearlike, adapted for piercing. First segment 

 of palpus of female with a hornlike process. Sternal plate broader 

 than long, with three pairs of setae; genitoventral plate small, with 

 indefinite anterior margin and a single pair of setae; anal plate 

 broadly rounded in front, with two anterior, paired setae and one 

 posterior, unpaired seta. Legs somewhat stout, second pair but 

 slightly, if at all, enlarged; one or more spines on all coxae, except 

 those of first pair. 



Tyjye species. — Dermanyssus musculi Koch, 1836. 



Remarks. — This genus is nearest Ophionyssus Megnin, 1884, but 

 differs from Megnin's genus in having a large posterior section of 

 the dorsal plate and three pairs of setae on the sternal plate instead 

 of two. 



CERATONYSSUS OCCmENTALIS, new species 



Plate 3, Figure 5 ; Plate 4, Figxtre 1 



Anterior dorsal plate about as long as, but much broader than, 

 posterior one, subtriangular, and bearing about 20 setae; posterior 

 dorsal plate broadest in front, pointed posteriorly, and bearing about 

 25 setae. Chelicerae very slender ; movable chela stouter and shorter 

 than the fixed one, which is very slender. Sternal plate much 

 broader than long, poorly sclerotized except for a heavy, arched 

 band along the posterior margin ; anterior corners acuminate ; front 

 pair of sternal setae marginal, last pair situated at apices of pos- 

 terior angles. Endopodal plates absent ; endopodal setae longer than 

 sternal setae. Genitoventral plate reduced, angulate, its single pair 

 of setae submarginal. Anal plate egg-shaped in outline; anus near 

 front margin; paired anal setae situated opposite center of anus, 

 unpaired seta situated about two thirds the distance from anus to 

 apex of anal plate. Legs moderate in length; posterior pair long- 



