[Illustration: FIG. 75.—Astacus leptodactylus (after Rathke, 1/3 nat. size).] In the north, _Astacus leptodactylus_ is met with in the rivers which flow into the White Sea, as well as in many streams and lakes about the Gulf of Finland. But it has probably been introduced into these streams by the canals which have been constructed to connect the basin of the Volga with the rivers which flow into the Baltic and into the White Sea. In the latter, the invading _A. leptodactylus_ is everywhere overcoming and driving out _A. nobilis_ in the struggle for existence, apparently in virtue of its more rapid multiplication.[26] [26] Kessler (Die Russischen Flusskrebse, l. c. p. 369–70), has an interesting discussion of this question. In the Caspian and in the brackish waters of the estuaries of the Dniester and the Bug, a somewhat different crayfish, which has been called _Astacus pachypus_, occurs; another closely allied form (_A. angulosus_) is met with in the mountain streams of the Crimea and of the northern face of the Caucasus; and a third, _A. colchicus_, has recently been discovered in the Rion, or Phasis of the ancients, which flows into the eastern extremity of the Black Sea. With respect to the question whether these Pontocaspian crayfishes are specifically distinct from one another, and whether the most widely distributed kind, _A. leptodactylus_, is distinct from _A. nobilis_, exactly the same difficulties arise as in the case of the west European {303} crayfishes. Gerstfeldt, who has had the opportunity of examining large series of specimens, concludes that the Pontocaspian crayfishes and _A. nobilis_ are all varieties of one species. Kessler, on the contrary, while he admits that _A. angulosus_ is, and _A. pachypus_ may be, a variety of _A. leptodactylus_, affirms that the latter is specifically distinct from _A. nobilis_. Undoubtedly, well marked examples of _A. leptodactylus_ are very different from _A. nobilis_. 1. The edges of the rostrum are produced into five or six sharp spines, instead of being smooth or slightly serrated as in _A. nobilis_. 2. The fore part of the rostrum has no serrated spinous median keel, such as commonly, though not universally, exists in _A. nobilis_. 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.