298 Birge — Notes on Cladocera. 



gether in these collections in much the same way as D. hyalina 

 and D. recrocurva are apt to do. Their numbers, however, are 

 more nearly equal, and in some bottles D. ?nicrocephala may be 

 the more numerous. 



In general form this species closely resembles D. longiremis. 

 The valves are perhaps somewhat more elongated, but still are 

 of a broadly eliptical form. The spine projects near the middle 

 of the shell and extends nearly straight backward. It is beset 

 with few and very small spinules. I see no difference between 

 D. longiremis and D. microcephala in these respects, although 

 Sars' description indicates one. 



The head is small ; its height is about one-half that of the 

 valves and its free projection is less than its height. It has a 

 slight keel on its dorsal side, which is wider on the anterior 

 edge. Seen from the side, the head is usually evenly rounded 

 in front but occasionally there is a trace of angulation. There 

 is never an indication of a projection or spine. The ventral 

 margin of the head is straight and rounds over smoothly at the 

 posterior angle. There is practically no rostrum. The sense- 

 hairs of the antennule project below the ventral margin of the 

 head. 



The antenna is moderately stout, having about the proportion 

 of that of D. hyalina. The seta of the basal joint of the ven- 

 tral ramus is about as large as its fellows. The eye is of mod- 

 erate size and of the type seen in D. hyalina, having a moder- 

 ately large amount of pigment. The material at command is 

 not so preserved as to show the other internal organs. 



The postabdomen is long and slender. It bears about nine 

 anal teeth. The caudal claws have the usual two spinules on 

 the anterior ventral side and are denticulate. The first and 

 second abdominal processes are slightly united at the base. 

 The second process is about one-half as long as the first. 

 Length 1.3-1.5 mm., including spine. 



The descriptions of this species given by Sars and Hellich do 

 not quite agree. The former speaks of a small macula nigra 

 ('63, p. 22.) while Hellich ('77, p. 37.) did not find that struc- 

 ture. He saw the species only once. I have been unable to see 

 the macula nigra, but the condition of the material is such that 



