NO. 1848. RECENT CLADOCERA FROM NEW ENGLAND— DOOLTTTLE. 163 



greatest height; ciUated. Upper angle sometimes accentuated, 

 especially in immature specimens, but never spined. Width from 

 side to side one-fourth length; suddenly narrowing anteriorly at 

 head, and more gradually posteriorly at the tubular part. Keel low, 

 of uniform height throughout. Upper and lower margins of the con- 

 cavity of sides thickened to form obvious ridges as seen from the 

 front, and traceable upon the sides of the test in lateral and dorsal 

 views. The opening or gape between the valves ventrally, rhom- 

 boidal or lozenge-shaped in anterior two-thirds, with sides of the figure 

 equal, straight; angles well defined. In posterior one- third, that is, 

 in the tubular portion, margins of valves ventrally parallel and 

 nearly touching. Test marked with delicate hexagonal meshes. 



Outline of head in side view semicircular; rostrum truncated. 

 The horizontal limb of fornix arising opposite the cervical sinus, 

 curving upward over base of second antenna and eye, then curving 

 downward with slight sinuosity almost to end of rostrum, very similar 

 to the course taken in Ophryoxus. Viewed from above, fornix broad 

 with parallel sides; from the front, narrowed. Eye moderate in size, 

 with few facets; macula nigra or ocellus nearly as large as eye, in base 

 of rostrum. First antenna or antennule cylmdrical, slightly smaller 

 in middle than at ends, the base swollen posteriorly; length ten times 

 average diameter; lateral seta one-fourth distance from base; three of 

 the terminal olfactory setae long, subequal, about two-thirds length 

 of antennule, and four times the length of the remaining six setae. 

 Each antennule with two longitudinal rows of seven or more equidis- 

 tant minute denticles; from side view one row appearmg near anterior 

 border, and one row near the posterior border. Second antenna long, 

 basal segment annulated, bearing upon a tubercle at the middle of 

 upper border two delicate setae with fleshy first joints; distally upon 

 lower border a strong spine. Rami armed typically for females of 

 the genus ; rami reaching to end of antennules or to first third of test ; 

 setae equaling the rami, reaching to posterior third of test. The 

 anterior lobe of labrum slender, backwardly curved; the posterior 

 lobe also slender, but anteriorly curved or hooked, almost touching 

 the anterior lobe, thus inclosing an oval space or f enestrum, opposite to 

 the mandibles, of the size and orientation of their grinding surfaces, 

 so that the mandibles oppose each other through the opening. Man- 

 dibles without noteworthy features. Maxillae with basal arm nar- 

 rowed distally; teeth three, set at an angle of 60° with base, subequal, 

 distal largest, and its convex border sparsely ciliated. 



Thoracic appendages or legs, six similar to those of other Lynco- 

 daphniidae, especially Ophryoxus gracilis G. O. Sars, yet differing 

 from OpJiryoxus and agreeing with all other genera of the family 

 in the absence of the maxillary process upon the first pair. The 

 spines and setae of the first leg are: upon the protopodite, a cluster 



