Marsh — North American Species of Diaptomus. 389 



are generally transparent, apparently never permanently col- 

 ored. Their bodies are elongated, and their appendages are 

 long and slender. The species of littoral habitat, or dwelling 

 in shallow water, have stouter bodies, are generally larger, 

 doubtless due to greater abundance of food, their appendages 

 are much shorter, and they are frequently highly colored. D. 

 shoshone is one of the most striking examples of these peculi- 

 arities. They are also very noticeable in D. leptopus and D. 

 sanguineus. 



STKUCTUKAL RELATIONSHIPS OF THE SPECIES. 



In this connection will be discussed only those peculiarities 

 iised for the separation of species. It is understood that this 

 covers only a small part of the subject, but it will be limited 

 with some precision to what is known. The characters used 

 for the determination of species are the following: 



1. The form and segmentation of the cephalothorax. 



2. The form and segmentation of the abdomen of the female, 

 Noticeable are the presence or absence of lateral spines or of a 

 caudal process on the first segment. 



3. The length of the antennae, and the armature of the last 

 three segments of the male ant«nna. This armature consists, 

 in the main, of hyaline lateral lamellae and of a process on the 

 antepenultimate segment which varies, being sometimes a hook, 

 more or less prolonged, sometimes an elongated affair which 

 may be armed Mdth teeth. 



4. The form of the fifth feet of the female. A fifth foot, 

 as can be seen by reference to the plates, consists of two basal 

 segments, an exopodite of two or three segments, the second 

 segment produced into a hook, and an endopodite of one or 

 two segments. This may vary in the following ways: 



a. In the form and size of the spine of the first basal seg- 

 ment. 



h. The exopodite may be two- or three-segmented, and, when 

 two-segmented, may have two or three lateral spines. 



c. The endopodite may be either one- or two-segmented, and 

 varies in length and in the size of the apical spines. 



