THE ETIOLOGY OF THE CRAYFISHES. 317 



the existing American crayfishes ; whether they are 

 Camhari or Astaci does not appear. But, in the lower 

 chalk of Ochti;up, in Westphalia, and therefore in a 

 marine deposit, Yon der Marck and Schliiter * have 

 obtained a single, somewhat imperfect, specimen of a 

 crustacean, which they term Astacus politus, and which, 

 singularly enough, has the divided telson found onl}- in 

 the genus Astacus. It would be very desirable to know 

 more about this interesting fossil. For the present it 

 affords a strong presumption that a marine Potamobine 

 existed as far back as the earlier part of the cretaceous 

 epoch. 



Such are the more important facts of Morphology, 

 Physiolog}^, and Distribution, which make up the sum 

 of our present knowledge of the Biology of Crayfishes. 

 The imperfection of that knowledge, especially as re- 

 gards the relations between Morphology and Distribution, 

 becomes a serious drawback w^hen we attack the final 

 problem of Biology, which is to find out wh}^ animals 

 of such structure and active powers, and so localized, 

 exist ? 



It would appear difficult to frame more than two 

 fundamental hypotheses in attempting to solve this pro- 

 blem. Either w^e must seek the origin of crayfishes in 

 conditions extraneous to the ordinarj^ course of natural 



* Neue Fische und Krebse aus der Kreide von Westphalcn. Pala^on- 

 tographica, Bd. XV., p. 302 ; tab. XLIV., figs. 4 and 5. 



