78 BRITISH PARASITIC COPEPODA. 



first three pairs are two-jointed, the fourth has only one- 

 jointed rami, and the abdomen consists of a single segment. 

 The form described below differs from both these groups, but 

 partakes to some extent of the characters of both. 



Nogaus ambiguus T. Scott. (Provisional name.) 

 (Plate XX, figs. 1-8.) 



1907. Nogagus ambiguus T. Scott. (117) Twenty-fifth Annual Report of 

 the Fishery Board for Scotland, Part III, p. 217, pi. xv, figs. 10-17 ( c? ). 



The cephalic shield in this form is of an oval outline, the 

 front is somewhat narrowly rounded but the frontal plates 

 are moderately large. The last two thoracic segments are 

 subequal, their width is about one-third of that of the 

 cephalic shield at its widest part, and the last segment is 

 truncated behind. The abdomen consists of a single small 

 subtriangular segment blunt-pomted at the apex; and the 

 caudal rami, which are short but moderately wide, are fur- 

 nished with tolerably long plumose setas. 



The antennules, which are of average size, are provided 

 with long plumose hairs. The antennas, mandibles, and 

 maxilhe are of the usual Galigus type. The first maxillipeds 

 are elongated, and are each armed with a loug and powerful 

 terminal claw which has a moderately stout seta at its base, 

 nearly as in Nogaus lunatus (Stp. & Liitk.), a species which 

 the present form resembles in some other particulars. The 

 second maxillipeds are short and very stout, and are each 

 fitted with a stout claw which forms, with the tuberculated 

 palm, a strong grasping organ. All the four pairs of swim- 

 ming legs are short and biramose ; in the first three pairs 

 both the outer and inner ramus are two-jointed and of nearly 

 equal length, and the end joints of both rami bear tolerably 

 long, densely plumose setae round the inner margin and end ; 

 there are also a few short spines on the outer margin. The 

 fourth pair are rather small, the inner ramus is biarticulate 

 as in the other three pairs, but the outer ramus is composed 

 of a single, somewhat club-shaped joint with three long 

 plumose setae round the distal end of the inner margin, and 

 with four spines — three small ones and a moderately large 

 terminal spine — on the exterior edge. Length about 5"5mm. 



Habitat. — Parasitic on a piked dog-fish, Squalus acanthias, 

 captured in the North Sea in 1902. 



This form has a somewhat close resemblance to N. lunatus 

 described by Steenstrup and Ltitken (' Bidrag til Kundskab/ 

 p. 389, pi. ix, fig. 17, 1861). 



