MORPHOLOGICAL AND GEOGRAPHICAL GROUPS. 3811 
be found that the parallel between the geographical and the 
morphological facts cannot be quite strictly carried out. 
Astacus torrentium, 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 crayfishes 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. 
Avother circumstance is still more remarkable. The 
West American crayfishes are but little more different 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-like process; while, in the 
females, the hinder edge of the penultimate thoracic 
