Side by side with _Eryma_, in the lithographic slates, there is a genus, _Pseudastacus_ (fig. 80, A), which, as its name implies, has an extraordinarily close resemblance to the crayfishes of the present day. Indeed there is no point of any importance in which (in the absence of any knowledge of the abdominal appendages in the males) it differs from them. On the other hand, in some features, as in the structure of the carapace, it differs from _Eryma_, much as the existing crayfishes differ from _Nephrops_. Thus, in the latter part of the Jurassic epoch, the Astacine type {344} 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 _Pseudastacus_ goes back as far, and that the common protastacine form is to be sought in the Trias. _Pseudastacus_ is found in the marine cretaceous rocks of the Lebanon, 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 _Parastacidæ_, 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 discovered, which possesses an especial interest as it is a true _Astacus_ (_A. politus_, Von der Marck and Schlüter), provided with the characteristic transversely divided telson which is found in the majority of the _Potamobiidæ_. If we arrange the results of palæontological 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. {345} SUCCESSIVE FORMS OF THE ASTACOMORPHOUS TYPE.