344 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES. 



was already distinct from the Homarine tj^pe, though 

 both were marine; and, since Eryma begins at least 

 as early as the Middle Lias, it is possible that Pseudas- 

 tacus goes back as far, and that the common protas- 

 tacine form is to be sought in the Trias. Pseudastacus 

 is found in the marine cretaceous rocks of the Leba- 

 non, but has not yet been traced into the Tertiary 

 formations. 



I am disposed to think that Ptieudastacus is comparable 

 to such a form as Astacus nigrescens rather than to any 

 of the Parastacidce, as I doubt the existence of the latter 

 group at any time in northern latitudes. 



In the chalk of Westphalia (also a marine deposit) a 

 single specimen of another Astacomorph has been dis- 

 covered, which possesses an especial interest as it is 

 a true Astacus {A. politus, Von der Marck and Schliiter), 

 provided with the characteristic transversely divided 

 telson which is found in the majority of the Pota- 

 mohiidcE. 



If we arrange the results of palseontological inquiry 

 which have now been stated ' in the form of a table 

 such as that which is given on the following page, 

 the significance of the succession of astacomorphous 

 forms, in time, becomes apparent. 



