A similar correspondence is exhibited, though less distinctly, when we consider the distribution of the genera and species of each group. Thus, among the _Potamobiidæ_, _Astacus torrentium_ and _nobilis_ belong essentially to the northern, western, and southern watersheds of the central European highlands, the streams of which flow respectively into the Baltic and the North Seas, the Atlantic and the Mediterranean (fig. 77, I.); _A. leptodactylus_, _pachypus_, _angulosus_, and _colchicus_, appertain to the Pontocaspian watershed, the rivers of which drain into the Black Sea and the Caspian (I.); while _Astacus dauricus_ and _A. Schrenckii_ are restricted to the widely separated basin of the Amur, which sheds its waters into the Pacific (II.) The _Astaci_ of the rivers of western North America, which flow into the Pacific (IV.), and the _Cambari_ of the Eastern or Atlantic water-shed (V.) are separated by the great physical barrier of the Rocky Mountain ranges. Finally, with regard to the _Parastacidæ_, the widely separated geographical regions of New Zealand (VIII.), Australia (IX.), Madagascar (XII.), and South America (VI. and VII.), are inhabited by generically distinct groups. But when we look more closely into the matter, it will {311} be found that the parallel between the geographical and the morphological facts cannot be quite strictly carried out. _Astacus torrentium_, as we have seen, inhabits both the British Islands and the continent of Europe; nevertheless, there is every reason to believe that twenty miles of sea water is an insuperable barrier to the passage of crayfishes from one land to the other. For though some crayfishes live in brackish water, there is no evidence that any existing species can maintain themselves in the sea. A fact of the same character meets us at the other side of the Eurasiatic continent, the Japanese and the Amurland crayfishes being closely allied; although it is not clear that there are any identical species on the two sides of the Sea of Japan. Another circumstance is still more remarkable. The West American crayfishes are but little more different from the Pontocaspian crayfishes, than these are from _Astacus torrentium_. On the face of the matter, one might therefore expect the Amurland and Japanese crayfishes, which are intermediate in geographical position, to be also intermediate, morphologically, between the Pontocaspian and the West American forms. But this is not the case. The branchial system of the Amurland _Astaci_ appears to be the same as that of the rest of the genus; but, in the males, the third joint (ischiopodite) of the second and third pair of ambulatory limbs is provided with a conical, recurved, hook-like process; while, in the females, the hinder edge of the penultimate thoracic {312} sternum is elevated into a transverse prominence, on the posterior face of which there is a pit or depression.[34]