466 Wisconsiii Academy of Sciences, Arts, and Letters. 



most noticeable point of difference, however, and the one that 

 at once separates this species from the others of the signicauda 

 group, is the form of the appendage of the antepenultimate 

 segment of the right male antenna. 



DIAPTOMUS TRYBOMI Lilljchorg. 



Plate XXVIII, figs. 1-4. 



1889. Diaptonius Tryhomi DeGuerne and Richa -d (Lillj.), p. 



58; pi. I, fig. 35; pi. II, fig. 6; pi. Ill, fig. 14; pi. 



IV, fig. 28. 

 1895. Diaptomus Tryhomi Herrick and Turner, p. 57 ; pi. 



VIII, fig. 17; pL IX, fig. 4; ph X, fig. 13. 

 1897. Diaptomus Tryhomi Schacht, p. 158 ; pi. XXXI, figs. 



1-5. 



''Of moderate size. Cephalothorax with greatest width at 

 about the middle; last two thoracic segments distinctly sepa- 

 rated, and the last of these as seen from above slightly produced 

 laterally, armed with two spines (one minute) on each side; 

 besides, on the right side of this segment appears a largo 

 dorsal appendage, triangular in form, mucronate at the apex, 

 and produced towards the right side. The first abdominal seg- 

 ment of the female is especially characteristic of the species (in 

 the male it is formed in the ordinary manner), as in D. sig- 

 nicauda asymmetrical, surpassing in length the rest of the ab- 

 domen. On the anterior part this segment is armed with a 

 short mucronate lateral process on either side, and on the 

 posterior part with a large triangular process extending almost 

 directly to the right and slightly acuminate at the apex. The 

 furcal rami are quite short, not twice as long as broad. 



"The first pair of antennae of the female are 25-segmented, 

 hardly reaching the base of the furca. The antepenultimate 

 segment of the prehensile antenna of the male is armed with a 

 straight and quite slender process, almost reaching the middle 

 of the penultimate segment, and provided on the outside to the 

 apex with small teeth. 



