THE PONTONIID^ 241 



fresh water, and, singular to relate, although they are in- 

 habitants of distant localities, several of which are oceanic 

 islands, yet all the species bear so close an affinity of form 

 that it is difficult to deteraiine one from the other by any 

 permanent character.' To explain this distribution, it is 

 suggested that ova may be carried in mud on the feet of 

 wading birds, and by this means species transferred from 

 one looaAity to another even over great distances. The in- 

 genious theory, already referred to, by which Mr. Thomas 

 Belt accounts in general for the wide distribution of fresh- 

 water species, does not seem applicable to those occupying 

 oceanic islands. Genera related to Atya are Atyephyra, 

 Brito Capello, 1867, for a Portuguese species, Paratya, 

 Miers, 1882, conditionally proposed for the Japanese 

 Ephyra? compressa of de Haan, Troglocmis, Dormitzer, 

 1853, which inhabits caves in Carinthia, and Evatya, S. I. 

 Smith, 1872, which belongs to Central America. 



Family 3. — Pontoniidce. 



The mandibles have a molar process and cutting edge 

 but no ' palp.' The first pair of trunk-legs are subequal, 

 slender, and chelate, the second unequal, one of the pair 

 very large in the male. 



Two genera are assigned to the family, both of which 

 occur in the Mediterranean. 



Pontonia, Latreille (1818), 1829, has the dorsal surface 

 flattened, and the rostrum dorsally flat. There is no 

 ocellus on the hind margin of the eye. The first antennas 

 have one of the two flagella bifid at the extremity, the 

 second have a short but strong scale. The fully developed 

 branchiae are only four or five pairs. It is not correct to 

 say that ' the telson carries no spine on the dorsal surface.' 

 Several species certainly have them. In Australian speci- 

 mens received from Mr. F. H. Haynes there are two pairs. 

 The apical margin of the telson in the Australian species 

 is fringed with six spines. The type species, Pontonia 

 custos (ForskSl), takes up its abode between the valves of 

 a Pinna, and is supposed to be referred to by Aristotle 

 when he says that a little shrimp (caridion) may take tho 



