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



Milne-Edwards it lias the carapace short and rounded (renflee), anteriorly armed with a 

 .short, robust, and depressed rostrum. 



The ophthalmopoda are cylindrical, prominent and very mobile. 



The first pair of antennae is very short and nearly similar to that of Palsemon. 

 The first joint of the peduncle is broad and lamellose on the outer side ; the two 

 succeeding joints are small, cylindrical, and terminate in two flagella, one of which is 

 bifid at the extremity. 



The second pair of antennae is inserted below and outside the first pair and carries 

 a broad and short scaphocerite. 



The second pair of gnathopoda (pates -machoires externes), according to Milne- 

 Edwards, is small and narrow in its entire length. 



The first two pans of pereiopoda x are didactyle. The first pair is subequal, slender, 

 and terminates in a well-formed but very small chela. Those of the second pair on 

 the contrary are very unequal, one being extremely large and the other small, especially 

 among the females. Sometimes the right and sometimes the left is the larger in 

 different specimens of the same species. The three succeeding pairs of pereiopoda are 

 of medium size, monodactyle, and terminate in a nearly rudimentary dactylos. 



The pleon is broad, especially in the females, and presents a conformation analogous 

 to that which exists in the genus Palsemon. 



It only remains to be noted that the telson carries no spine on the dorsal surface. 



The branchiae are well developed ; there are only five on each side, those belonging 

 to the oral appendages being rudimentary, and the somites of the pereion carry only 

 single pairs. 



Dana's description corresponds with that of Mdne-Edwards, excepting that he says 

 the outer maxillipedes (second pair of gnathopoda) are suboperculiform. 



There is but one specimen of this genus in the collection and that is much damaged, 

 all the pereiopoda excepting the greater chela being lost, and the posterior somites, 

 pleon, and rhipidura are wanting. 



Observations. — This genus corresponds closely with Typton, Costa, but there are 

 several points of difference. The dorsal surface is depressed and flattened, instead of 

 being elevated and arcuate. The rostrum is dorsally flat, instead of being laterally 

 compressed. There is no ocellus on the posterior margin of the ophthalmus as there is 

 in Typton. The first pair of antennas has the inner flagellum bifurcate at the extremity, 

 whereas it is single in Typton ; in both the stylocerite is reduced to a rudimentary 

 condition. The second pair of antennae in both genera is small and feeble ; in Pontonia 

 the scaphocerite is well developed but short and strong, whereas in Typton it is reduced 

 to a rudimentary condition, forming a small membranous scale. The mandibles in 



1 In Milne-Edwards' description the paragraph runs " Les pates des quatre premieres paires sont didactyles," which 

 from the context is evidently a misprint for " deux paires." 



