THE ORIGIN OF CRAYFISHES. 331 



land and by other changes in physical geography. And, 

 indeed, under these circumstances, the freshwater prawns 

 thomselves might become so much modified, that, even if 

 the descendants of their ancestors remained unchanged 

 in structure and habits in the sea, the relationship of the 

 two might no longer be obvious. 



These considerations appear to me to indicate the di- 

 rection in which we must look for a rational explanation 

 of the origin of crayfishes and their present distribution. 



I have no doubt that they are derived from ancestors 

 which lived altogether in the sea, as the great majority of 

 the MysidcB and many of the prawns do now ; and that, of 

 these ancestral crayfishes, there were some which, like 

 Mysis oculata or Penaus brasiliensis,re9idi\y adapted them- 

 selves to fresh water conditions, ascended rivers, and took 

 possession of lakes. These, more or less modified, have 

 given rise to the existing crayfishes, while the primitive 

 stock would seem to have vanished. At any rate, at the 

 present time, no marine crustacean with the characters 

 of the Astacidce is known. 



As crayfishes have been found in the later tertiariea 

 of North America, we shall hardly err in dating the 

 existence of these marine crayfishes at least as far back 

 as the miocene epoch ; and I am disposed to think that, 

 during the earlier tertiary and later mesozoic periods, 

 these Crustacea not only had as wide a distribution as 

 the Prawns and Pencei have now, but were differentiated 

 into two groups, one with the general characters of the 



