THE ETIOLOGY OF THE CRAYFISHES. 317 



the existing American crayfishes; whether they are 

 Camban 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, somewhat imperfect, specimen of a 

 crustacean, which they term Astaciis polit us, 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, especially 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 why 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 we must seek the origin of crayfishes in 

 conditions extraneous to the ordinary course of natural 



* Neue Fische und Krebse ana der Kreide von Westphalen. Palaeon 

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



