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



There can, I think, be little doubt that the three specimens taken are merely different 

 sexes of one and the same species. It is, however, curious, that during the expedition the 

 only females should be taken off New Zealand, while the only male taken should have 

 been four thousand miles distant. 



Family Astacid^:. 



The o-enera of this family are all inhabitants of fresh water rivers, lakes, and 



ponds. 



The podobranchise are invariably six, and well developed. The plumes are attached 

 to the mastioobranchial plates in their entire length. Posterior to each of these a sub- 

 lunate process tipped with hairs is articulated. The anterior three pairs of pereiopoda 

 are chelate. The first pair is largest. 



The term Potamobius was first used by Dr. Leach, in 1819, 1 for the genus Astacus, 

 and Astacus? for the marine genus to which Milne-Edwards, in his Histoire des Crustaces, 

 afterwards gave the name of Homarus, by which it has ever since been recognised. 



At that time Milne-Edwards retained the name of Astacus for the terrestrial or fresh- 

 water species, by which it has since been universally known; and although, as Dana* 

 writes, " Leach has undoubted priority and exhibited his usual discrimination in proposing 

 the subdivision of the old genus," yet to adopt now the name of Potamobius instead of 

 Astacus would only create great confusion. It is to be regretted that to acknowledge 

 Dr. Leach's claim would not only have this result, which he himself would deprecate, but 

 would introduce terms not likely to be generally accepted. 



The Astacidse of the northern hemisphere have the first pair of pleopoda modified in 

 the male for sexual purposes, and the dactylos of the first or large chelate pair of pereio- 

 poda has the outer margin straight or indexed. They have never more than one pleuro- 

 branchial plume on each side fully developed. 



There are two genera in this division, Astacus, in which a pleurobranchial plume 

 is attached to the posterior somite of the pereion, and Cambarus, which has none. These 

 are confined to the northern hemisphere, and may be found in Europe, Asia, and 

 America. 



Those that belong to the southern hemisphere have the first pair of pleopoda wanting, 

 and the dactylos of the first or large chelate pair of pereiopoda has the outer margin 

 convex. There are always one or more pleurobranchial plumes on each side. 



1 Potamobia fluviatilis, Leach, Sam. Eut. u. Comp., p. 95, 1819. Mr. Walter Faxon, in his Eevision of the 

 Astacidae, 1885, attributes this generic name to Sowerbyin the continuation of Leach's Malacostraca PoJophthalmia 

 Britannia?, No. 8, xviii, xix. Tab. xxxiv. fig. 1, 1875. Adam White in Cat. Brit. Mus., 1850, uses it ; and L. Agassiz in 

 the Nomenclatoris Zoologici, Index Universalis, gives Potamobia, Leach, Crust., 1818. 



2 Astacus gwmmarus, Leach, Sam. Ent. u. Comp., vol. vii. p. 398. 



3 Dana, U.S. Explor. Exped., p. 532. 



