AQUATIC INVERTEBRATE FAUNA OF WYOMING AND MONTANA. 241 



hairs moderately robust, the first segment not longer than the second. No emargi- 

 nation separating head from body, but the dorsal surface very slightly sinuate there. 

 Abdominal processes all distinct, anterior much the longest. Uaudal claw with 

 accessory teeth, about six in number, the, three proximal the largest. Anal spines 

 about ten. Posterior spine of the shell given off from the middle of the valves, in 

 the adult female a third to a half as long as the valve. Shell moderately coarsely 

 marked in quadrangular areas, the lower and dorsal margins spiuulose from the 

 middle backwards. Length 2 millimeters, depth 1 millimeter. 

 Heart Lake, Yellowstone Park, Wyoming. 



Dapbnia thorata, n. sp. 



This species belongs to the hyalina group, and may possibly be entitled only to 

 varietal rank. The distinctness and constancy of its characters, however, in collec- 

 tions made by us in Flathead and Swan lakes in western Montana, and the number 

 of minor points in which it differs from hyalina, as most recently described, lead 

 me to distinguish it here as a specific form. 



It is oval in outline; the long and slender posterior spine is placed at or a trifle 

 above the middle; the length of the head is about a third that of the valves of the 

 shell excluding the spine, and there is no trace of dorsal emargination between head 

 and body. The head is narrowed toward the base and elongated forward in a way 

 to give it the outline of a high bell jar with a flaring base. Its front is broadly 

 and regularly rounded, its ventral margin usually conspicuously concave and closely 

 like the dorsal, although occasionally the head is straight or convex beneath. The 

 posterior margin is either straight or slightly concave, and the beak stands free from 

 the front margin of the valves, and by its extension downward not only covers the an- 

 ten use but reaches clearly beyond the tips of the sensory hairs. The eye is of medium 

 size, placed far back of the front of the head and equidistant from the tip of the beak 

 and the dorsal junction of the head an d body. The pigment speck is of moderate size, 

 placed directly behind the eye, and much farther from it than from the posterior mar- 

 gin of the head. 



The antenna} are moderately stout, entirely smooth except for inconspicuous trans- 

 verse rows of minute appressed hairs upon both peduncle and rami, and a row of 

 short, tooth-like spinules at the distal end of each segment. The swimming hairs are 

 rather slender, the second joint commonly decidedly shorter than the first. 



Pornices slight, arising above and a little behind the eye and terminating directly 

 behind the antennae, above the bases of which they project but slightly. The lower 

 margin of each valve is set with the usual spiuules almost to the beak, and the dor- 

 sal margin is similarly armed for a distance in front of the spine about equal to half 

 the length of the latter. The valves are marked off by fine lines into large quadrate 

 meshes. 



The dorsal abdominal processes rise separately, the two anterior, however, in 

 immediate contact at their bases. . The first of these is decidedly the longer, but the 

 third process is distinct, although low. The anal setae are two-jointed, the second 

 joint the shorter. The abdomen is regularly narrowed backwards, and the anal 

 groove is provided with twelve to fifteen teeth on each side, commonly the" latter 

 number. The terminal claws are without accessory comb. The intestinal cceca are 

 short, not longer than the diameter of the eye, and extend directly forward. 

 F. C. B. 189116 



