72 THE VOYAGE OF H.M.S. CHALLENGER. 



The first pair of antennae lias the first joint short, broad and conical, the second and 

 third slender, the terminal joint reaching beyond the extremity of the second pair. 



The second pair of antennae have five cusps on the anterior margin and two on the 

 inner, and is nearly free from cilia. The second or first free joint is armed on the outer 

 margin with three points or teeth and three on the anterior margin, while the longi- 

 tudinally-oblique ridge is slightly dentate. 



The first pair of pereiopoda is larger than the others, and the posterior has a large 

 sharp, curved tooth (fig. 3") projecting backwards from the posterior angle of the coxa! 

 ridge of the podal socket of the ventral plate. 



The three anterior somites of the pleon are without pleopoda, those of the second and 

 third being either accidentally absent, or not yet developed. Those of the fourth and 

 fifth somites consist of two long sub-foliaceous branches on a tolerably long stalk. The 

 inner ramus carries a single stylamblys. 



Length, 19 mm. (075 in.). 



Habitat. — The specimen from which this description is taken was dredged off Cape 

 Verde, but neither station nor depth are recorded. 



Observations. — Two other specimens were taken off Gomera, one of the Canary 

 Islands, in 75 fathoms of water, associated with Arctus pugmzeus, with which they have 

 several points in common, that suggest from their association that they might be the 

 males of that small species. They agree in the general form of the second pair of 

 antenna?, the absence of hairs being attributable to sexual variation or to having been 

 worn away by friction and use. Both have minute specks of pigment, more especially on 

 the plates of the second pair of antennae, but they are considerably more conspicuous 

 on Arctus 'pygmseus than on Arctus immaturus. The absence of dorsal ornamentation 

 might also be attributed to sexual difference, but I am not aware of such variation to 

 any great degree in the genus, though in Ibaccus the separation is quite as apparent and 

 important. 



The reason why I came to the opinion that Arctus immaturus is the young of some 

 other form rather than the male of Arctus pygmasus, with which it almost corresponds in 

 size, being but a little smaller, depends upon the structure and form of the pleopoda. Iii 

 Arctus pygmmis these appendages agree in structure with those of the adult females of 

 other known species ; but in Arctus immaturus they agree more with the pleopoda of 

 the males of other types than with those of any species of the genera. 



The two sub-foliaceous plates, thickly fringed with long ciliated hairs, the inner of 

 which carries a stylamblys, vary in the three specimens, and in one, apparently the most 

 mature, it is half the length of the branch to which it is attached, and the extremity is 

 covered with small cincinnuli, perhaps the rudimentary condition of hairs which in the 

 females become the important points for the attachment of the ova. 



