The materials which have, up to the present, time been collected are too scanty to permit of the tracing out of all the details of the genealogy of the crayfish. Nevertheless, the evidence which exists is perfectly clear, as far as it goes, and is in complete accordance with the requirements of the doctrine of evolution. Mention has been made of the close affinity between the crayfishes and the lobsters—the _Astacina_ and the _Homarina_; and it fortunately happens that these two groups, which may be included under the common name of the _Astacomorpha_, are readily distinguishable from all the other _Podophthalmia_ by peculiarities of their exoskeleton which are readily seen in all well-preserved fossils. In all, as in the crayfish, there are large forceps, followed by two pairs of chelate ambulatory limbs, while {339} the succeeding two pairs of legs are terminated by simple claws. The exopodite of the last abdominal appendage is divided into two parts by a transverse suture. The pleura of the second abdominal somite are larger than the others, and overlap those of the first somite, which are very small. Any fossil crustacean which presents all these characters, is certainly one of the _Astacomorpha_. The _Astacina_, again, are distinguished from the _Homarina_ by the mobility of the last thoracic somite, and the characters of the first and second abdominal appendages, when they are present; or by their entire absence. But it is so difficult to make out anything about either of these characters in fossils, that, so far as I am aware, we know nothing about them in any fossil Astacomorph. And hence, it may be impossible to say to which division any given form belongs, unless its resemblances to known types are so minute and so close as to remove doubt. For the present purpose, the series of the fossiliferous rocks may be grouped as follows:—1. Recent and Quaternary. 2. Newer Tertiary (Pliocene and Miocene). 3. Older Tertiary (Eocene). 4. Cretaceous (Chalk, Greensand and Gault). 5. Wealden. 6. Jurassic (Purbeck to Inferior Oolite). 7. Liassic. 8. Triassic. 9. Permian. 10. Carboniferous. 11. Devonian. 12. Silurian. 13. Cambrian. [Illustration: FIG. 80.—A, _Pseudastacus pustulosus_ (nat. size). B, _Eryma modestiformis_ (× 2). Both figures after Oppel.] 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.