AQUATIC INVERTEBRATE FAUNA OP WYOMING AND MONTANA. 243 



half of it straight. A slight beak is formed just below the sensory antennae, the latter 

 being attached at a small angular einargiuatiou at the posterior angle of the head. 

 From this emargination the posterior margin of the head passes directly upwards in 

 a broad and gentle curve. The eye is very large, placed at the very front of the head. 

 Its longitudinal diameter is contained but once in the head behind the eye. Sensory 

 antenna slightly clavate, slightly expanded at the middle, its length equal to the 

 vertical diameter of the eye. In front of the terminal group of sensory hairs is a long 

 termiual spine, nearly as long as the antenna itself, slightly curved backwards and seg- 

 mented at the middle. Accessory hair distant from end, but a little below the middle. 



Length without spine, 1.4 millimeters; depth 0.9 millimeter; spine, 0.33 millimeter. 

 A single hairy dorsal abdominal process, as in pulex. 



Yellowstone Lake and other waters of Yellowstone Park. 



Daphnia dentifera, n. sp. (Plate xxxvn, Fig. 2.) 



This species is broad oval in form, has a long beak and a very large eye, a poste- 

 rior spine placed high up, and in the male and young female a prominent angle on the 

 dorsal outline between heart and eye. 



The head is broadly rounded, with eye close to the front margin. The foruices are 

 short, rising above and behind the eye and extending backwards a little beyond the 

 base of the antenme, where they form a prominent angle. Thence a slight lateral keel 

 of the valve is continued downwards and backwards a distance about equal to the 

 length of the fornix. The lower margin of the head is broadly concave, the beak pro- 

 duced, projecting as far as the ends of the sensory hairs. The large eye, with numer- 

 ous lenses, is contained not more than twice in the distance from eye to beak, its 

 diameter a little greater than that of the base of the antenna at its insertion. Pigment 

 speck of moderate size, circular, immediately behind the eye and nearer to that than 

 to the posterior margin of the head. 



The head is slightly crested, and the crest, extending backward to the heart, 

 rises over the antenna? in a prominent, nearly rectangular process, still more acute 

 in the young, the tip of which is commonly truncate and bears two or three teeth 

 inclining forward. In the egg-bearing female this process is reduced to a mere 

 obtuse angle, or, in the last generation (that bearing the ephippinm), disappears 

 entirely. In young adults this dorsal angle is midway between the eye and the heart, 

 but when fully developed it is on a line drawn from the anterior margin of the valve 

 to the middle of the base of the antenna. The seta} of the antennae are all two-jointed, 

 the basal joint distinctly the longest. The posterior spine of the carapace is long, 

 slender, and weak, and is commonly contained three or four times in the head and body 

 without the spine. 



The margins of the valves are set below and behind with slender thorns, as is 

 also the posterior spine, these thorns extending forward a little distance upon the 

 dorsal margin of the shell. The curved spines bordering the anal furrow are thirteen 

 in number; the caudal claws are without accessory teeth; the surface of the shell is 

 marked with quadrangular areolations. 



The first and second dorsal abdominal processes are about equal in length and 

 arise in immediate contact, the anterior turning forward and the posterior backward. 



Mature female 1.8 millimeters long by 1 millimeter deep. 



The male of this species is smaller than the female; the head is smaller and nar- 

 rower, the form is more nearly elliptical, and the dorsal angle is as prominent as in 



