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 Ochtrup, in Westphalia, and therefore in a 

 marine deposit, Von der Marck and Schliiter * have 

 obtained a single, somew^hat imperfect, specimen of a 

 crustacean, which they term Astacus politus, and which, 

 singularly enough, has the divided telson found only 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, 

 Physiology, and Distribution, which make up the sum 

 of our present knowledge of the Biology of Crayfishes. 

 The imperfection of that knowledge, especiall}- as re- 

 gards the relations between Morphology and Distribution, 

 becomes a serious drawback when we attack the final 

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

 of such structure and active j)owers, and so localized, 

 exist ? 



It would appear difQcult to frame more than two 

 fundamental h3^potheses in attempting to solve this pro- 

 blem. Either we must seek the origin of crayfishes in 

 conditions extraneous to the ordinar}^ course of natural 



* Neue Fische und Krebse aus der Kreide von Westphalen. PalEeon- 

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



15 



