44 TWELFTH ANNUAL REPORT. 



ing of the vas deferens close behind the terminal claws; macro 

 short and blunt, length 1 mm. This species is very near the next, 

 but differs in several particulars. It forms the transition to the 

 next, which is the extreme of the genus in a direction converse to 

 that pursued by the S. armata. 



Sp, 5. Scapholeberis angulata, Herrick. 

 (Plate B. Fig. 9. Plate T. Fig. 7.) 



American Naturalist, 1S8J. 



Form as in the above, but comparatively larger; valves quadran- 

 gular, anterior margin strongly arched; head short, only slightly 

 concave below the eyes; the beak is as in S. nasuta, but seems to 

 be directed more nearly directly downward than in that species. 

 The antennules are long and resemble those of Simocephalus, The 

 pigment fleck is square and rather large; the antennae are of the 

 usual size. The reticulated areas are as in the other species. The 

 post-abdomen is more as in Daphnia, not so squarely truncate and 

 with five to seven large teeth; the first foot has one elongated 

 jointed seta; the posterior angle of the shell has no spine, at most 

 there is a somewhat prominent acute angle, the inner shell layer 

 is armed at this point with some elongated teeth as in the corres- 

 ponding situation in Simocephalus. On the whole, there is a 

 similiarity to that genus in this as well as in the previous species. 

 S. nasuta has a short spine and elongated pigment fleck; the pres- 

 ent species has a squarish but rather large fleck and no spine; the 

 post-abdomen has a greater number of spines than any other 

 species. South of Tennessee river, in Alabama and Mississippi. 

 . The species of this genus are predominatingly American, four 

 out of the five being found in the United States; the fifth, more- 

 over, is more often regarded a variety of one of the others; in fact, 

 the absence of S. cornuta from America is one of the most impor- 

 tant supports of the specific independence of the two forms. All 

 the species delight in disporting themselves near the surface in 

 sunny weather. 



IV. — Genus Simocephalus. 



Although a very well circumscribed group, this genus passes 

 into the next rather directly by means of S. macrothroides. The 

 connection on the other hand seems to be by the way of Scaphole- 

 beris, though there is a rather broad separation between even 

 Scapholeberis angulata and any known Simocephalus. The en- 



