344 DISTRIBUTION AND ETIOLOGY OF THE CRAYFISHES. 
was already distinct from the Homarine type, 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 Pseudastacus is comparable 
to such a form as Astacus nigrescens rather than to any 
of the Parastacide, 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- 
mobiide. 
If we arrange the results of paleontological 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. 
