344 TJISTRIBUTIOX AND ETIOLOGY OF THE CRAYFISHES. 



Avas already distinct from the Homarine type, though 

 hoth were marine ; and, since Eryma begins at least 

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

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

 tacine form is to be sought in the Trias. Pseudastarus 

 is found in the marine cretaceous rocks of the Leba- 

 non, but has not yet been traced into the Tertiary 

 formations. 



I am disjDosed to think that Pseudastacus is comparable 

 to such a form as Astacus nigrescens rather than to any 

 of the Parastacid(e, as I doubt the existence of the latter 

 group at anj' time in northern latitiides. 



In the chalk of "Westplialia (also a marine deposit) ^ 

 single specimen of another Astacomorph has been dis- 

 covered, which possesses an especial interest as it is 

 a true Astacus {A. politus, Yon der March and Schliiter), 

 provided with the characteristic transversely divided 

 telson ■which is found in the majority of the Pot<i- 

 vtohiidfP. 



If we arrange the results of paheontological 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 apjiarent. 



