of the Pontoniidae. 379 



(a) PericUmenes. 



(b) Coralliocaris, Harpilius. 



(c) Anchislus, Pontonia, Conchodytes, 



(d) Typton. 



(a) PericUmenes stands out at once as the most primitive 

 genus of the family. Its Palanion -hke form ot body, com- 

 pressed dentate rostrum (except in P. aurantiacus) , dactyles 

 of the last three pairs of legs straight and not swollen at the 

 base, narrow third maxillipeds, and its free-living habits, all 

 point to this conclusion. 



(l>) In the second group the compressed dentate rostrum of 

 PericUmenes usually becomes broader and shallower, and is 

 often shorter ; but the dentations, though showing signs of 

 reduction, are generally present, at least on the upper surface, 

 and the free end is pointed both in dorsal and in lateral view, 

 and is not curved downwards. The last joint of the second 

 maxilliped is as broad as — usually broader than — the pre- 

 ceding joint, and the antepenultimate joint of the third 

 maxilliped is broadened, though not to a great extent, in 

 Coralliocaris, The dactyles ot the last three legs are short, 

 stout, curved, and without small teeth on the underside. The 

 two genera of the group differ between themselves in the 

 attachment of the last joint of the second maxilliped to the 

 penultimate, which takes place terminally in Coralliocaris, 

 but laterally on the inner side in Harpilius. A further 

 distinction is afforded by the third maxilliped, which in 

 Coralliocaris has the antepenultimate joint of moderate 

 breadth and the last two joints about equalling it in this 

 dimension. In Harpilius } on the other hand, the last two 

 joints are narrow and the preceding joint is broad. Lastly, 

 the dactyles of the hinder three pairs of legs are provided in 

 the former genus with a large basal protuberance on the 

 underside. This protuberance is wanting in Harpilius. 



(c) In the third group the rostrum is more or less depressed 

 at its base, but distally is deep and strongly compressed 

 (except in some species of Pontonia). Dentations are absent 

 or restricted to the tip, which is bent downwards and in side 

 view rounded or diminishing abruptly to a point. The 

 penultimate joint of the second maxilliped is broader than the 

 last joint and bears it terminally. The third maxilliped, 

 fairly narrow in Anchistus, is broad in the antepenultimate 

 joint in Pontonia and in all the joints in Conchodytes. The 

 dactyles of the last three pairs of legs are straight in some of 

 the species and of varying degrees of curvature in others. 

 They may or may not bear teeth on the underside, and in 



