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



The pleopoda and rhipidura correspond with those of the genus Palsemon. 



Observations. — The solitary specimen in the collection is only about a third of the 

 length of that described by Dana, but it appears to have its parts completely developed. 

 It corresponds so closely with the description of Dana's species that I have no doubt of 

 its identity with it, and I attribute certain differences to the young condition of the 

 specimen under examination ; but it is difficult to completely examine it without 

 injuring it. In the Challenger specimen the ophthalmus is much larger in proportion 

 than in Dana's figure, and, with the utmost care, I have not been able to detect the 

 hepatic tooth on the carapace, nor has Dana shown it in his figure or alluded to it in 

 his description of this species, although he mentions it in his generic diagnosis. The 

 dactylos of the posterior three pairs of pereiopoda is biunguiculate, a fact that is over- 

 looked by Dana. 



Bithynis, Philippi. 



Bithynis, Philippi, Wiegmann's Archiv f. Naturgesck., Jahrg. xxvi. p. 161, 1860. 

 Macrolirachium, Spence Bate, Proc. Zool. Soc. Lond., p. 363, 1868. 

 Palsemon (pars), Dana, U.S. Explor. Exped., Crust., p. 584. 



„ Stimpson, Proc. Acad. Nat. Sci. Philad., p. 110, 1860. 



„ (Division 2), Milne-Edwards, Hist. Nat. Crust., tom. ii. p. 395. 



Like Palsemon, but differing in having a tooth on the hepatic region and no tooth 

 corresponding with the second antennas (the "spina branchiostegiana " of Stimpson), 

 in having the carpos of the second pair of pereiopoda long and cylindrical, and this 

 appendage developed in the adult to a greater length than that of the entire animal, and 

 more or less unequal, and in having the pleon shorter in proportion to the length of the 

 carapace. 



Geographical Distribution. — Species of this genus are more or less present in the 

 fresh-water streams of tropical Asia, America, and Africa ; when they have been taken in 

 the sea it has been only at the mouths of the rivers they are found to inhabit. 



Mr. Kingsley ' says that this form is far from being uncommon in salt water, and 

 instances several species, as Bithynis spinimanus, Bithynis grandimanis, Bithynis 

 jamaicensis, Bithynis forceps, &c. Milne-Edwards says that Bithynis ornatus, Bithynis 

 carcinus, and other long-armed forms are found in different parts of the Indian Ocean, 

 and that Bithynis jamaicensis inhabits the Antilles. 



There is undoubtedly a peculiarity belonging to this group that distinguishes it at 

 once from the typical form of Palsemon, and although there is evidence of specimens 

 having been taken in the sea, yet in several instances they are supposed to be marine, 

 because the locality to which they belong has been alone recorded, without any special 

 notice of their having been found in fresh water as their normal habitat. Bithynis 



1 Bull. Essex Inst., vol. x. p. 66. 



