of the Fishery Board for Scotland. 



155 



correspondingly short or long. Now, in regard to the forms under con- 

 sideration, it will be observed that the females of Dr. Basset-Smith's 

 Lepeophtheirus obscurus have the abdomen moderately long and two- 

 jointed, whereas Dr. Baird's male has a "small abdomen" consisting of 

 one segment. It may be that, notwithstanding this discrepancy, the two 

 forms belong to the one species ; but I am inclined to think that Dr. 

 Baird's own suggestion — that his Lepeophtheirus obscurus is the male of 

 Lepeophtheirus hippoglossi — is after all the correct explanation of the 

 difficulty. 



Genus Trebius, Kroyer (1838). 



This genus resembles Lepeophtheirus in its general configuration, in the 

 absence of lunulas on the frontal plates and in the possession of bifurcated 

 palpi. On the other hand, the most distinctive and obvious difference 

 between Trebius and Lepeophtheirus is that in the former the fourth pair 

 of thoracic feet are furnished with two branches instead of only one branch, 

 as in the case of the latter. 



The only species belonging to this genus is the one described below. 



Trebius caudatus, Kroyer. (PI. VI., figs. 20-26.) 



1838. Trebius caudatus, Kr. Naturh. Tidsskrift (1838), R. i., 



vol. ii., p. 30, PI. I., fig. 4. 

 1850. Trebius caudatus, Baird, op. cit., p. 280, PI. XXXIII., 



figs. 3, 4. 



In the female of this species the cephalic shield, which is about as long 

 as the entire remaining portion of the thorax, is nearly oval, and rather 

 longer than broad. The last segment of the thorax is sub-cylindrical, its 

 breadth being equal to about four-fifths of the length ; the posterio-lateral 

 angles are rounded and fringed on the posterior aspect with three small 

 but stout spines. The abdomen is long and slender, and composed of two 

 segments ; a slight constriction is also observable towards the distal end 

 of the second segment, which has in some specimens the appearance of an 

 additional joint. The caudal segments are small. 



The antennae (posterior antennae) are armed with strongly-hooked 

 terminal claws (fig. 22). The mandibles are somewhat similar to those 

 of Lepeophtheirus, but the end-joint appears to be rather stouter than the 

 preceding one, and is distinctly serrated on the inner margin (fig. 24). 

 The palpi are bifurcated, but the inner branch of each palpus is rather 

 shorter than the other. The sternal fork is small, and the branches are 

 simple and comparatively short (fig. 23). In the fourth pair of thoracic 

 feet the basal joints are stout and comparatively short ; both of the three- 

 jointed branches are also short, but the outer ones are armed exteriorly 

 with five moderately stout marginal spines, and interiorly with plumose 

 marginal setae. The inner branches are also setiferous, but they want 

 the exterior marginal spines. 



The male (fig. 21), which is scarcely half the length of the female, has 

 the fourth thoracic segment very small. The abdomen consists of two 

 segments, and is of moderate length ; the two segments are somewhat 

 unequal, the first being rather shorter than the other. The caudal segments 

 are slightly longer than those of the female. 



The length of the female specimen figured is about four ^tenths of an 

 inch (10mm.). In another specimen of about the same length the 

 abdomen measured 3mm.; while in a somewhat larger one, the entire 

 length of which was 11mm., the abdomen measured 4mm. in length, or 

 fully two-thirds the entire length of the animal. 



K 



