1082 THE VOYAGE OF H.M.S. CHALLENGE].!. 



grounds for separating the genus Aora from Microdeuteropus. We have seen that the 

 females of [the] two are almost undistinguishable ; and if Aora be divided from Micro- 

 deuteropus because the toothdike projection proceeds from the meros [third joint] and 

 not the carpus [wrist], M. Websterii must in justice have a similar distinction conferred 

 upon it, because in that species the toothdike projection does not spring from either 

 meros or carpus, but from the hand." Mr. Chilton in 1885 definitely unites the genera 

 Aora and Microdeutopus, and would, it may be presumed, make Autonoe also a synonym 

 of Aora. Boeck adopted the other alternative suggested by Mr. Norman, and allowed 

 Gammarus longipes to have the distinction of a separate generic name on account of the 

 hand of the first gnathopods ; his definition is as follows : — 



"First Gnathopods larger than the Second; the fifth joint in both sexes forming 

 the hand, which is stronger in the male than in the female. 



"The Third Uropods with the outer ramus longer than the inner. 



" In other points almost as in the genus Microdeutopus." 



Practically the generic character must be reduced to the description of the first 

 gnathopods, since in the description which Boeck gives alike of Autonoe longipes, 

 Liljeborg, and of his own Autonoe plumosa, it is clear that the difference in length 

 between the two rami of the third uropods is insignificant. None the less I am much 

 more doubtful than I formerly was of the expedience of combining the three genera 

 Aora, Microdeutopus, and Autonoe, since the character of the first gnathopods in the 

 male of Aora is so peculiar, that, as more and more species in the group become known, 

 there will be a continual tendency, I imagine, to draw the -4ora-forni apart from the 

 other two, and then the severance also of those two becomes, as Norman points out, a 

 logical consequence. 



The definition of Microdeutopus, to which Boeck refers in defining both Aora and 

 Autonoe, is as follows : — 



" Upper Antennas longer than the Lower ; the third joint of the peduncle short. 



" First Gnathopods larger than the Second ; the wrist of the male very dilated, 

 produced at the lower hinder angle ; the fifth joint or hand narrower than the wrist and 

 together with the finger forming a two-jointed thumb (una cum ungve pollicem 

 2articulatum formanti); the fifth joint in the female very dilated and forming the hand. 



"The First and Second Perseopods with the finger shorter than the fifth joint. 



"The Third Uropods with the inner and outer ramus almost equal in length." 



Autonoe philacantha, n. sp. (PI. CX.). 



Rostrum scarcely perceptible, lateral lobes of the head small, acute, lower angles still 

 more acute ; the posterodateral angles of the first three pleon-segments rounded, especially 

 those of the third segment. 



