670 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxi. 



except the thoracic legs. Genital segment enlarged, but never much 

 more than half the size of the carapace. Abdomen elongate; anal 

 laminie long and narrow. Furca and first maxillae both present. All 

 the swimming legs biramose; rami of first pair two-jointed, of the 

 other pairs three jointed, except in exilis^ where the fourth pair has a 

 two-jointed endopod. Egg strings as in the Caliginte. Adults active, 

 both sexes swimming about freel}'.. The young reported by Kroyer 

 (1863) and Olsson (1869) to pass through a chalimus stage in which 

 they are attached b}' a frontal tilament, the remains of which can be 

 plain!}' seen in the median incision of the frontal plates in later stages 

 of development (fig. 2). 



This subfamily stands as a connecting link between the Caligina? and 

 the Euryphorina?. In its development it is almost identical with the 

 Caliginje, but in its morphology it is radically different from them and 

 more closely reseml)les the Eurj-phorina?, though difi'ering from the 

 latter also in many important particulars. At present it is composed 

 of the single genus TreMu.s. 



It would seem at first as if this genus could be included either with 

 the Caliginse or the Euryphorinte, rather than separated from them 

 both. 



Kro^'er, Steenstrup and Liitken, and Heller place it with the Cali- 

 ginre, while Gerstaecker puts it with his " Nogagina" (Eur^'phorinae). 

 The following considerations have seemed sufficient to the author to 

 warrant its separation in a subfamily by itself: 



A. If it Avere included with the Caliginse — 



1. It would be the only genus having more than a single free thorax 

 segment. In all the other genera the three anterior segments of the 

 thorax are fused with the head to form a carapace, which has the same 

 general shape in every genus and the same arrangement of grooves 

 and areas. Moreover, the fusion is complete, and despite the grooves 

 there is very little motion, if an}', between the difierent areas. Here 

 in Trehius only two thorax segments are fused with the head, and 

 there are several important differences in the grooves and areas. 



For instance, the thoracic area, which in the Caliginse includes the 

 three anterior thoracic segments, is here restricted to the second seg- 

 ment alone and has been so much shortened as to become transversely 

 semilunar. Again, the grooves separatine' the lateral areas extend 

 forward to the very bases of the first antenna?, a condition found in 

 none of the Caligina?. 



The short transverse grooves also which separate the cephalic from 

 the thoracic portions of the lateral areas extend to the very edge of 

 the carapace and form there well-defined notches or incisions. But 

 more than all this, the fusion of the different areas is not so complete 

 as to prevent considei'a))le freedom of motion along the various 

 grooves. This is especially true of the second thorax segment, which. 



