"8C PROF. T. H. HUXLEY ON THE CLASSIFICATION I JlUlP 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 geo- 

 graphical divisions of the Crayfishes. Thus, all the Crayfishes of the 

 northern hemisphere are Potamobiidae, aud 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 Palasarctic region, with the Neotropical region as far 

 as Guatemala and the West Indies. 



In the southern hemisphere, Astacopsis, Chceraps, and Engans 

 are confined to the Australian region, Paranephrops to New Zea- 

 land and the Fijis ; while the South-American Parasiticus 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 the 

 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 Salmonidse, for ex- 

 ample, corresponds pretty closely with that of the Potamobiida?, 

 though the range of the Salmonidse extends less far to the south 

 in North America, and a little further, namely, as far as Algeria in 

 the Old World. Again, the Salmouidee 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 Retropinna, there is no true Sal- 

 monoid in the southern hemisphere ; but, as Dr. Giinther has 

 pointed out, the Haplocbitonidae and the Galaxiadae, which stand in 

 somewhat the same relation to the Salmonidae as the Parastacidse 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 Haplocbitonidae, 

 and the Galaxiadae are singular among Teleostean fishes for the em- 

 bryonic 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 podobranchiae. 



With the exception of one or two species in Algeria and Asia 

 Minor, the Salmonoids and their allies are wanting in the whole of 



