3. The posterior end of the post-orbital ridge is still more distinct and spiniform than in A. nobilis.
4. The abdominal pleura of A. leptodactylus are narrower, more equal sided, and triangular in shape.
5. The chelæ of the forceps, especially in the males, are more elongated; and the moveable and fixed claws are slenderer and have their opposed edges straighter and less tuberculated.
But, in all these respects, individual specimens of A. nobilis vary in the direction of A. leptodactylus and vice versâ; and if A. angulosus and A. pachypus are varieties of A. leptodactylus, I cannot see why Gerstfeldt’s conclusion that A. nobilis is another variety of {304} the same form need be questioned on morphological grounds. However, Kessler asserts that, in those localities in which A. leptodactylus and A. nobilis live together, no intermediate forms occur, which is presumptive evidence that they do not intermix by breeding.
No crayfishes are known to inhabit the rivers of the northern Asiatic watershed, such as the Obi, Yenisei, and Lena. None are known[27] in the sea of Aral, or the great rivers Oxus and Jaxartes, which feed that vast lake; nor any in the lakes of Balkash and Baikal. If further exploration verifies this negative fact, it will be not a little remarkable; inasmuch as two[28], if not more, kinds of crayfishes are found in the basin of the great river Amur, which drains a large area of north-eastern Asia, and debouches into the Gulf of Tartary, in about the latitude of York.
Japan has one species (A. japonicus), perhaps more; but no crayfish has as yet been made known in any part of eastern Asia, south of Amurland. There are certainly none in Hindostan; none are known in Persia, Arabia, or Syria. In Asia Minor the only recorded locality is the Rion. No crayfish has yet been discovered in the whole continent of Africa.[29] {305}
[27] It would be hazardous, however, to assume that none exist, especially in the Oxus, which formerly flowed into the Caspian.
[28] A. dauricus and A. Schrenckii.
[29] Whatever the so-called Astacus capensis of the Cape Colony may be, it is certainly not a crayfish.