Now the oldest known member of the group of the {341} decapod Podophthalmia to which the Astacomorpha belong occurs in the Carboniferous formation. It is the genus Anthrapalæmon—a small and very curious crustacean, about which nothing more need be said at present, as it does not appear to have special affinities with the Astacomorpha. In the later formations, up to the top of the Trias, podophthalmatous Crustacea are very rare; and, unless the Triassic genus Pemphix is an exception, no Astacomorphs are known to occur in them. The specimens of Pemphix which I have examined are not sufficiently complete to enable me to express any opinion about them.
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.
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.
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.