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



outer, and between this pair of pleopoda are two teeth, one on each side of the ventral 

 median line. 



The posterior pair has the branches subequal in length and but little longer than the 

 telson, the basal joint articulates with the sixth somite, in a deep excavation at the 

 posterior angle. 



Observations. — This species so nearly corresponds with Nauticaris marionis in all 

 conveniently accessible parts that I have placed it in the same genus, although it does 

 not possess the small peculiar spinule at the postero-inferior angle of the sixth somite, 

 which may be only of specific value. Only one specimen of this species appears to 

 have been taken during the voyage. 



Hetairus, 1 n. gen. 



Kostrum horizontal, armed on the upper and lower margins with teeth, flanked at the 

 base with a supraorbital tooth, below the orbit with an antenna! tooth, and at the fronto- 

 lateral angle with a small point. 



The ophthalmopoda are large, pyriform, and furnished with a small ocellus. 



The first pair of antennas is short and biflagellate. 



The second pair carries a large scaphocerite and a flagellum as long as the animal. 



The mandibles consist of the molar process, a psalistoma, and a two-jointed 

 synaphipod. 



The first two pairs of siagnopoda correspond generically with those of Hippolyte and 

 Spirontocaris. The third pair carries a two-lobed mastigobranchia. 



The first pair of gnathopoda carries a mastigobranchia and a rudimentary podo- 

 branchial plume. 



The second pair of gnathopoda carries a rudimentary mastigobranchia, but neither 

 branchia nor basecphysis. 



The first three pairs of pereiopoda carry a rudimentary mastigobranchia, but the 

 posterior two pairs are without it. The first pair is robust and chelate. The second is 

 slender, long and minutely chelate, the carpos being seven-articulate. 



The branchiae consists of five pleurobranchial plumes, which may with the other parts 

 be tabulated as follows : — 



Pleurobranchise, . . . 1 1 1 1 1 



Arthrobranchise, . . . 



Podobranchiie, 

 Mastigobrancliite, 



Geographical Distribution. — Atlantic Ocean. 



Observations. — This genus has a peculiar interest. In many points it possesses 



1 From kra'i^o;, an associate. 



