752 PROF. T. H. HUXLEY ON THE CLASSIFICATION [June 4, Oil the Classification and the Distribution of the Cray-fishes. By T. H. Huxley, Sec. U.S., V.P.Z.S. [Received May 23, 1878.] I. Introduction, p. 752. II. The Modifications of the Branchiae in the Crayfishes, p. 756. 1. The branchia 1 of Astacus, p. 756. 2. The branchiae of Cambarus, p 763. 3. The branchiae o.f Asfaeoptis, p. 764. 4. The branchiae of C/usraps, p. 768. 5. The branchiae of Engaus, p. 769. 6. The branchiae of Paranephrops, p. 770. 7. The branchial' of Parastacus, p. 771. 8. The branchiae of Astacoides, p. 773. III. The Classification of tbe Crayfishes, p. 775. IV. The Distribution of the Crayfishes considered in relation to their morphological differences, p. 786. I. Introduction. The dismemberment of the genus Astacus of the older naturalists, down to the time of Fabricius, was commenced by Leach, who sepa-rated the Norway Lobster as the type of a new genus, Nephrops 1 . Milne-Edwards advanced a step further by establishing the genus Hotnarus for the Lobsters, and leaving only the freshwater Astaci, or the proper Crayfishes, in Astacus' 2 . The later proposal of Leach, to use Astacus for the Lobsters, and to give a new generic name (Potamobius) to the freshwater Cray-fishes, would have had the advantage of retaining the primitive signification of atrraicos. But Potamobius had already been used in another sense ; and the change introduced by Milne-Edwards is so generally adopted that it would be confusing to attempt any further alteration. Guerin 3 next proposed to distinguish the Astacus madagascari-ensis of Audouin and Milne-Edwards, as Astacoides, from the other Crayfishes ; and Erichson, in his valuable Monograph of the group 4 , adopts Astacoides for the Madagascar and some of the Aus-tralian forms, and establishes the new genera Cambarus, Chceraps, and Engceus. In Cambarus and Chceraps the number of the branchiae is taken into account as an important generic character. In 1842 s Mr. Adam White described some Crayfish from New Zealand, for which be constituted a new genus, Paranephrops, under the impression that the New-Zealand form approximated to the genus Nephrops. Mr. Wood-Mason 6 has since "denied the existence of any special relationship between the New-Zealand 1 Leach, Trans. Linn. Soc. xi. 344. 2 'Histoire Naturelle des Crustaces,' 1837. 3 Revue Zoologique, 1839. 4 " Uebersicbt d. Gattung Astacus," Archiv fur Naturgeschichte, Bd. 6. 5 Gray's ' Zoological Miscellany.' See also Dieffenbach's 'New Zealand,' 1843, vol. ii. p. 267. 6 P-oc. Asiat. Soc. Bengal. 1876, p. 4.