The case is altered when we reach the Middle Lias. In fact this yields several forms of a genus, _Eryma_ (fig. 80, B), which also occurs in the overlying strata almost up to the top of the Jurassic series, and presents so many variations that nearly forty different species have been recognised. _Eryma_ is, in all respects, an Astacomorph, and so far as can be seen, it differs from the existing genera only in such respects as those in which they differ from one another. Thus it is quite certain that Astacomorphous _Crustacea_ have existed since a period so remote as the older part of the Mesozoic period; and any hesitation in admitting this singular persistency of type on the part of the crayfishes, is at once removed by the consideration of the fact that, along with _Eryma_, in the Middle Lias, prawn-like _Crustacea_, generically identical with the existing _Penæus_, flourished in the sea {342} and left their remains in the mud of the ancient sea bottom. [Illustration: FIG. 81.—_Hoploparia longimana_ (2/3 nat. size).—_cp_, carapace; _r_, rostrum, T, telson; XV., XVI., first and second abdominal somites; 10, forceps; 20, last abdominal appendage.] _Eryma_ is the only crustacean, which can be certainly ascribed to the _Astacomorpha_, that has hitherto been found in the strata from the Middle Lias to the lithographic slates; which last lie in the upper part of the Jurassic series. In the freshwater beds of the Wealden, no _Astacomorpha_ are known, and although no very great weight is to be attached to a negative fact of this kind, it is, so far, evidence that the _Astacomorpha_ had not yet taken to freshwater life. In the marine deposits of the Cretaceous epoch, however, astacomorphous forms, which {343} are known by the generic names of _Hoploparia_ and _Enoploclytia_, are abundant. The differences between these two genera, and between both and _Eryma_, are altogether insignificant from a broad morphological point of view. They appear to me to be of less importance than those which obtain between the different existing genera of crayfishes. _Hoploparia_ is found in the London clay. It therefore extends beyond the bounds of the Mesozoic epoch into the older Tertiary. But when this genus is compared with the existing _Homarus_ and _Nephrops_, it is found partly to resemble the one and partly the other. Thus, on one line, the actual series of forms which have succeeded one another from the Liassic epoch to the present day, is such as must have existed if the common lobster and the Norway lobster are the descendants of _Erymoid_ crustaceans which inhabited the seas of the Liassic epoch.