466 Wisconsin 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 Lilljeborg. 
Plate XXVIII, figs. 1-4. 
1889. Diaptomus Trybomi DeGuerne and Picha xL (Lillj.), p. 
58; pi. I, fig. 35; pi. II, fig. 6; pi. Ill, fig. 14; pi. 
IV, fig. 28. 
1895. Diaptomus Trybomi Herrick and Turner, p. 57; pi. 
VIII, fig. 17; pi. IX, fig. 4; pi. X, fig. 13. 
1897. Diaptomus Trybomi 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. 
