672 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxi. 



in the Caligin^e. This is the most important difference, and furnishes, 

 in the author's opinion, a sufficient reason for excluding the genus 

 from the Euryphorina?. 



Genus TREBIUS Kroyer. 



Carapace usually oval or elliptical and quite strongl}^ arched dor- 

 sally. Third thorax segment short and wide; free, but attached to the 

 posterior margin of the carapace in such a way as to complete a tho- 

 racic area somewhat like that in the Caligina\ The grooves, however, 

 are arranged differently and consist of a semiellipse at the posterior end 

 of the carapace, a longitudinal groove on either side extending foi*- 

 ward to the lateral sinus behind the base of the first antenna, and a 

 transverse groove extending outward on either side from this longi- 

 tudinal groove to a notch in the edge of the carapace. 



The body of the copepod is capable of more motion along these 

 grooves, particularly the semiellipse, than in the genera of the Caligin^. 



Fourth segment more or less elongate, abruptl}^ narrowed anteriorly 

 and posteriori}^ with its sides projecting stronglj^ at the center over 

 the bases of the fourth legs. 



Genital segment considerably smaller in the male and showing two 

 pairs of legs, one on the sides and the other at the posterior corners. 

 Egg-strings as in Caligus; eggs small and numerous. 



Mouth-tiibe long and wide and distinctl}' hinged at the center; mouth- 

 opening terminal and heavily fringed with hairs. 



Mandibles slender, slightly curved, and toothed on the inner margin 

 onl3^ Second maxilla? long and pointed, articulate; either simple or 

 slightl}^ bifurcate at the tips. First maxillipeds stouter and the second 

 pair weaker than in the Caliginae, thus eliminating much of the differ- 

 ence between the two appendages. 



{trehlus^ the name of a parasite in Juvenal, Satire V.) 



ONTOGENY. 



The life history of this genus is v^ery similar, so far as known, to that 

 of the Caliginre. The following summary is taken from the works of 

 various authors, chiefly Kroyer (1863) and Olsson (1869), supplemented 

 b}^ original research: 



Nothing is known of the nauplius and metanauplius stages; the 

 youngest individual so far obtained was a small chalimus found by 

 Kroyer amongst the preserved material he examined. But this 

 chalimus is so similar to those found among the Caligina? as to leave 

 little doubt that the earlier stages are equally" similar, and that when 

 found they will differ simpl}^ in detail and not in any of the essential 

 characters. 



Kroyer's chalimus specimen (fig. 1) was about 1.5 mm. long and of an 

 elongated oval fdrm. Carapace two-fifths the entire length, as wide as 



