28 A REVISION OF THE BRITISH SPECIES OF 



of Diaptomus, needs no description further than to refer to a 

 few salient characters which distinguish it from the rest of the 

 genus. 



The animal is stout and robust in build, — the female consider- 

 ably larger and heavier than the male. The posterior extremity 

 of the thorax is truncated, and in the female the lateral angles 

 are strongly produced, and form mucronate cusps ; in the male 

 the thorax is more tapered posteriorly, and the angles are only 

 slightly produced. The abdomen in the male is five-jointed, 

 slender, and of nearly equal width throughout ; in the female 

 the segments are reduced to three, the first occupying half the 

 length of the abdomen, and having its margins produced later- 

 ally so as to form two large truncated processes; the second 

 segment is extremely small, and the last somewhat larger. The 

 anterior antennae are composed of twenty-five joints, and, in the 

 female, reach, when reflexed, as far as about the middle of the first 

 abdominal segment ; in the male they are very nearly as long as 

 the entire body of the animal. The right anterior antenna of 

 the male is, of course, geniculated, but the antepenultimate joint 

 is destitute of any special appendage or process. The basal 

 joint of the posterior maxilliped has the anterior distal angle 

 much produced, rounded, minutely crenulated, and bearing a 

 series of very minute cilia, below which are three or four larger 

 hairs, and, at the very extremity, one very long and stout seta. 

 The fifth pair of feet in the female are alike on both sides, and 

 composed of a two- jointed basal portion (protopodite) and two 

 branches, each consisting of two joints. The first joint of the 

 outer branch (exopodite) is simple, the second forms a stout, 

 slightly curved claw, with dilated base, from which spring one 

 long spine-like seta and two very much shorter ones : the inner 

 branch (endopodite) is equal in length to the first joint of the 

 exopodite, and is composed of two distinct joints, the second of 

 which bears at its apex three setce, one as long as the entire 

 branch, another extremely short (almost imperceptible), and 

 a third of intermediate length. In the male the inner branch 

 of the right side is very small, slender, composed of two simple 

 joints, and reaches scarcely to the middle of the penultimate 



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