570 CRUSTACEA. 



nence on the under side of the tarsus is wanting, and the outer max- 

 illipeds are peculiar, the second joint being broad, and the following- 

 narrow, as in the true Pontoniae, but the latter two together longer 

 than the second, unlike Pontonia. 



In another genus (our Anchistia), differing but little from the pre- 

 ceding, the outer maxillipeds are slender throughout ; the tarsus is 

 also slender and nearly straight ; the body is not depressed. 



In all the groups which have been mentioned, the mandible is 

 without a palpus, and the inner antennae have but two flagella. 

 although one of them is sometimes bifid at apex. 



The next group (Palsemonella) is between the last mentioned and 

 Palsemon. The form is that of Palaemon ; the inner antennae have 

 two flagella, with one bifid at tip ; the tarsi are slender ; the man- 

 dibles have a rudimentary palpus of two joints. They pass into 

 Palaemon through those species of the latter genus, which have two 

 of the three flagella of the inner antennas united nearly to their ex- 

 tremities. 



From this survey of these genera and their relations, it is evident 

 that Pontonia and Palaemon pertain to the same natural group, and 

 both are removed by important characters from the Alpheinse. 



GENUS PONTONIA, Latreilk. 



Corpus depressum. Oculi parvi. Maxillipedes externi sub-operculi- 

 formes, articulo 2do latiusculo, latiore et lonyiore quam duo sequentes. 

 Rostrum depressum, breve. Antennae internee flagellis duobus confectxe. 

 Tarsi uncinati. 



Body depressed. Eyes small. Outer maxillipeds suboperculiform, 

 second joint rather broad, broader and longer than the two follow- 

 ing. Rostrum depressed, short. Inner antennae ending in two 

 short flagella. Tarsi uncinate. 



The Pontonia tyrrliena is the type of this genus. We refer to this 

 group another species, found, like that, within the shell of a bivalve 

 mollusc. The shell is more opaque and thicker than in the Pontonia 

 macrophthalma and allied ; the legs are shorter ; the eyes very much 

 smaller ; the outer maxillipeds very different ; the tarsi destitute of 



