630 THE VOYAGE OF H.M.S. CHALLENGEE. 



Hvpijomedon trigonicus, n. sp. (PI. IX.), 



In many respects tliis species shows a very close resemblance to Hippomeclon 

 kergueleni (Miers), although there are peculiarities which have induced me, after some 

 wavering, to keep it distinct. In the present form the postero-lateral angle of the third 

 pleon-segment is but little, instead of greatly, curved upwards. Of the fourth pleon- 

 segment the proximal portion is very convex, the depression being distal, so that the end 

 of the segment forms a raised angular apex rising above, instead of forming a continuous 

 curve with, the following segment. 



It originally appeared to me that the dorsal depression in HipjMmedon kergueleni 

 was in a marked manner proximal, and in the form now under consideration conspicuously 

 distal, but I find in this and many other species of Amphipods that the dorsal appearance 

 of the fourth pleon-segment is very essentially altered according as the pleon happens to 

 be more or less extended or flexed. In the state of extension the proximal portion 

 of this segment often telescopes far into the third segment, sometimes completely hiding 

 a dorsal depression, and at others making such a depression appear proximal, when in 

 regard to the whole dorsal length of the segment it is in fact distal or central. 



The other differences between the two forms may be judged of from the following 

 account. 



Upper Antennie. — The peduncles comparatively slender, the first joint longer in 

 proportion to its breadth than in the form already described ; the flagellum consisting of 

 eleven joints, the first equal in length to between two and three of those succeeding it ; 

 the secondary flagellum of three joints, of which the first is not quite so long as the first 

 of the primary. 



Lower Antenna. — Gland-cone prominent, third joint equal in length to the fifth ; 

 fourth joint decidedly longer than either ; flagellum nine-jointed. 



The Mouth-Organs appear to be in close agreement with those previously described. In 

 the specimen examined there were fewer teeth on the apex of the palp of the first maxiUa. 



First Gnathopods. — There seems to be no difference of importance except in the shape 

 of the side-plates, which are here of less regular form, shorter and stouter, outdrawn in 

 front below. At the beginning of the palm of the hand there are two spines. 



Second Gnathopods. — In this species the wrist is a little plumj^er distally, hand and 

 wrist both densely furred, but the distal spine-armature of the hand both at front and 

 back less important than in the other species. 



Second Perseopods. — What may be called the shank of the side-plate is here some- 

 what longer in proportion to the broad lower portion. 



Third PersBopods. — First joint more contracted below, and with margins more curved, 

 so as to have an oval rather than the square appearance presented in the other species ; 

 the third and fourth joints longer compared with their breadth. 



