[35] According to Dr. Günther their southern range is similarly limited by the Asiatic Highlands. But they abound in the rivers both of the old and new worlds which flow into the Arctic sea; and though those on the western side of the Rocky Mountains are different from the Eastern American forms, yet there are species common to both the Asiatic and the American coasts of the North Pacific. Thus, these fish avoid South Africa, as the crayfishes {316} do; but I am not aware that any member of the group is found in Madagascar, and thus completes the analogy. * * * * * The preservation of the soft parts of animals in the fossil state depends upon favourable conditions of rare occurrence; and, in the case of the _Crustacea_, it is not often that one can hope to meet with such small hard parts as the abdominal members, in a good state of preservation. But without recourse to the branchial apparatus, and to the abdominal appendages, it might be very difficult to say whether a given crustacean belonged to the Astacine, or to the closely allied Homarine group. Of course, if the accompanying fossils indicated that the deposit in which the remains occur, was of freshwater origin, the presumption in favour of their Astacine nature would be very strong; but if they were inhabitants of the sea, the problem whether the crustacean in question was a marine Astacine, or a true Homarine, might be very hard to solve. Undoubted remains of crayfishes have hitherto been discovered only in freshwater strata of late tertiary age. In Idaho, North America, Professor Cope[36] found, in association with _Mastodon mirificus_, and _Equus excelsus_, several species, which he considers to be distinct from {317} the existing American crayfishes; whether they are _Cambari_ or _Astaci_ does not appear. But, in the lower chalk of Ochtrup, in Westphalia, and therefore in a marine deposit, Von der Marck and Schlüter[37] have obtained a single, somewhat imperfect, specimen of a crustacean, which they term _Astacus politus_, and which, singularly enough, has the divided telson found only in the genus _Astacus_. It would be very desirable to know more about this interesting fossil. For the present it affords a strong presumption that a marine Potamobine existed as far back as the earlier part of the cretaceous epoch. [36] On three extinct _Astaci_ from the freshwater Tertiary of Idaho. Proceedings of the American Philosophical Society, 1869–70. [37] Neue Fische und Krebse aus der Kreide von Westphalen. Palæontographica, Bd. XV., p. 302; tab. XLIV., figs. 4 and 5. * * * * *