Marsh — North American Species of Diaptomus. 409 



Lateral swelling of first abdominal segment 

 distinct, sicilis, Ashlandi, oregonensis, 

 pallidum. Reighardi, mississippiensis, Lin- 

 toiii, spatulocrenatus. 

 (These species cannot be satisfactorily separ- 

 rated by the characteristics of the fe- 

 male, although the limits of size will 

 help somewhat.) 



The lateral spines of the abdomen are distinct, 



The exopoditei is distinctly three-segmented, 



the endopodite is two-segmented, Bakeri 



The exopodite is two-segmented, the endopodite 

 one-segmented, the second segment of the 

 exopodite has three spines, conipedatus, 

 clavipes, Tyrelli, leptopus. 

 (These cannot be separated by the char- 

 acteristics of the female.) 



The second segment of the exopodite has two 

 spines, siciloides, sanguineus, Birgei, nova- 

 mexicanu^. 



DIAPTOMUS OREGONENSIS Lllljeborg. 



Plate XV, figs. 1, 4, 7. 

 1889. Diaptomus oregonensis DeGuerne and Richard, p. 53; 



pi. II, fig. 5; pi. Ill, fig. 8. 

 1893. Diaptomus oregonensis Marsh, p. 200; pi. IV, figs. 4, 5. 

 1895. Diaptomus oregonensis Marsh, p. 8 ; pi. VII, fig. 5. 

 1895. Diaptomus oregonensis Herri ck and Turner, p. 72 ; pi. 



IV,figs. 7-12;pl.IX,fig. 3. 

 1897. Diaptomus oregonensis Schaeht, p. 151 ; pi. XXIX, figs. 



1 and 2. 

 1906. Diaptomus pygmaeus Pearse, p. 244 ; figs. 1, 2 and 3. 



Of medium size. Cephalothorax widest at the middle. The 

 first segment of the cephalothorax exceeds in length the three 

 following segments. The last segment is slightly produced 

 laterally and armed with two minute spines. 



The first segment of the female abdomen equals in length 

 the rest of the abdomen. It is slightly expanded laterally, 

 and sometimes bears two very minute lateral spines; thes3, 

 however, are not always present. The second segment is shori r 

 than the third, and the third shorter than the furcal rami. 

 The length of the furcal rami is twice their width; they are 

 finely ciliate on the inner margin. 



