THE ORIGIN OF CRAYFISHES. 331 
land and by other changes in physical geography. And, 
indeed, under these circumstances, the freshwater prawns 
themselves 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 Myside and many of the prawns do now; and that, of 
these ancestral crayfishes, there were some which, like 
Mysis oculata or Peneus brasiliensis, readily 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 Astacide is known. 
As crayfishes have been found in the later tertiaries 
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 ag 
the Prawns and Penei have now, but were differentiated 
into two groups, one with the general characters of the 
