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Show 786 PROF. T. H. HUXLF.Y ON THE CLASSIFICATION [Julie 4, IV. THE DISTRIBUTION OF THE CRAYFISHES CONSIDERED IN RELATION TO THEIR MORPHOLOGICAL DIFFERENCES. From what has been said above, it will be obvious that there is a remarkable correspondence between the morphological and the geographical divisions of the Crayfishes. Thus, all the Crayfishes of the northern hemisphere are Potamobiidae, and all those of the southern hemisphere are Parastacidae. In the northern hemisphere, again, the Astaci are Eurasiatic and West-American, while the Cambari are characteristic of the North-American region east of the Sierra Nevada-in other words, of the river-basins which flow into the Gulf of Mexico and the West Atlantic. The Astacine region nearly answers to the Palaearctic province of Mr. Sclater, minus the southern shores of the Mediterranean, and plus Western North America; while the Cambarine region takes in most of the Palaearctic region, with the Neotropical region as far as Guatemala and the West Indies. In the southern hemisphere, Astacopsis, Chceraps, and Engceus are confined to the Australian region, Paranephrops to New Zealand and the Fijis ; while the South-American Parastacus is distinct from either of these, though closely allied with the Australian forms; and the peculiarity of the Madagascar fauna is vindicated by Astacoides. Thus, if we were to establish provinces of distribution on tbe Crayfishes alone, they would bear only a partial resemblance to those based on the association of terrestrial animals. On the other hand, if we compare the distribution of the Crayfishes with that of the freshwater fishes, there are, as might be expected, some curious points of resemblance. The distribution of the Salmonidae, for example, corresponds pretty closely with that of the Potamobiidae, though the range of the Salmonidae extends less far to the south in North America, and a little further, namely, as far as Algeria in the Old World. Again, the Salmonidae to the east of the Rocky- Mountains are, for the most part, distinct from those to the west, while the genus Onchorhynchus is, like Astacus, common to both the Asiatic and the American shores of the North Pacific. With the singular exception of Betropinna, there is no true Sal-monoid in the southern hemisphere; but, as Dr. Gunther has pointed out, the Haplocbitonida? and the Galaxiadae, which stand in somewhat the same relation to the Salmonidae as the Parastacida? do to the Potamobiidae, represent the Salmonidae in the fresh waters of New Zealand, Australia, and South America. It is worthy of remark that the Salmonidae, the Haplochitonida?, and the Galaxiadae are singular among Teleostean fishes for the embryonic character of their female reproductive organs, which have no oviducts-just as, among the Podophthalmous Crustacea, the Crayfishes are distinguished by the undifferentiated character of their podobranchia?. With the exception of one or two species in Algeria and Asia Minor, the Salmonoids and their allies are wanting in the whole of |
