214 GAMMARID.E. 



A MP II I POD A . PHOXIDES. 



NATATOJifA. 



Genus— IS^A. 



Isrra. Milne-Edwards, Ann. Sci. Nat. t. xx. p. 380. Hist, des Crust, 

 t. iii. p. 26. Spence Bate, Ann. Nat. Hist. 2 ser. xix. p. 142, 

 1857. Cat, Amph. Crust. Brit. Mus. p. 122. White, Pop. Hist. 

 Brit. Crust, p. 179. 



Generic character. Superior auteuase having a secondary 

 appendage. Gnathopoda subchelate; second pair the larger. 

 Pereiopoda subchelate. Posterior pair of pleopoda biramous. 

 Telson cylindrical, single. 



In this genus .the cephalon is not much produced 

 anteriorly. The eyes are two. The superior antennae 

 are longer than the inferior, and furnished with a se- 

 condary appendage. The arms are subchelate ; the 

 hand of the second pair is larger than that of the first ; 

 all the legs are likewise subchelate, the distal extremity 

 of the hands being broadly dilated on the side towards 

 which the joints bend. The coxae of the four or five 

 anterior pairs of legs are deep. The last pair of caudal 

 appendages are double-branched, and the central tail- 

 piece is round and solid, the alimentary canal probably 

 opening at its posterior extremity. 



Dana has established a subfamily to receive this genus, 

 together with Anisopus of Templeton, under the name of 

 Isaance, based upon the subchelate condition of the walk- 

 ing legs. We do not perceive any advantage to be de- 

 rived from this arrangement, since Isaa and Anisopus 

 (which latter we consider to belong either to the genus 

 Amphithoe or Sunamphithoe) cannot be embraced in the 

 same subfamily; and it will be found that there are other 

 species possessing a more or less perfectly marked sub- 

 chelate condition of the walking legs, which cannot be 

 associated with Is(Pa. 



