MORPHOLOGICAL AND GEOGRAPHICAL GROUPS. 311 



be found that the parallel between the geographical and the 

 morphological facts cannot be quite strictly carried out. 



Astacus torrentiiim, as we have seen, inhabits both 

 the British Islands and the continent of Europe ; never- 

 theless, there is every reason to believe that twenty 

 miles of sea water is an insuperable barrier to the 

 passage of craj^fishes from one land to the other. For 

 though some crayfishes live in brackish water, there is 

 no evidence that any existing species can maintain them- 

 selves 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 difi*erent from 

 the Pontocaspian crayfishes, than these are from Astacus 

 torrentium. On the face of the matter, one might there- 

 fore 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-lil^e process ; while, in the 

 females, the hinder edge of the penultimate thoracic 



