JVb.2.] PACKARD ON TERTIARY CRAYFISH. 397 



seen that tlie facts of distribution suggest the hypothesis that they 

 must have been so, at least up to this time. 



'• Thus, with respect to the aetiology of the crayfishes, all the known 

 facts are in harmony with the requirements of the hypothesis that they 

 have been gradually evolved in the course of the Mesozoicand subsequent 

 epochs of the world's history from a primitive Astacomorphous form." 

 (The Crayfish, p. 341-34G.) 



It will thus be seen that the discovery of an apj)arently fresh water 

 Canibarus in the Green River beds of Western Wyoming, which are 

 supposed to be Lower Eocene strata, fills up a break in the geological 

 series hitherto existing between the Cretaceous and Pliocene crayfishes, 

 and shows that the dynasty of fresh water crayfish now so powerfully 

 developed in the United States began its reign during the early Tertiary 

 period. 



