a very ancient continental area, the oldest indigenous population of which, in all probability, is directly descended from that which occupied it at the beginning of the tertiary epoch. If Parastacine Crustacea inhabited the southern hemisphere at this period, and subsequently became extinct as marine animals, their preservation in the freshwaters of Australia, New Zealand, and the older portions of South America may be understood. The difficulty of the absence of crayfishes in South Africa * remains; and all that can be said is, that it is a difficulty of the same nature as that which confronts us when we compare the fauna of South Africa in general with that of Madagascar. The population of the latter region has a more ancient aspect than that of the former; and it may be that South Africa, in its present shape, is of very much later date than Madagascar.

With respect to the second point for consideration, it is to be remarked that, in the temperate regions of the world, the crayfishes are by far the largest and strongest of any of the inhabitants of freshwater, except the Vertebrata; and that while frogs and the like fall an easy prey to them, they must be formidable enemies and competitors even to fishes, aquatic reptiles, and the smaller aquatic mammals. In warm climates, however, nɔt only the large prawns which have been mentioned, but Atya

But it must be remembered that we have as yet everything to learn respecting the fauna of the great inland lakes and river systems of South Africa.

THE DISTRIBUTION OF CRABS AND CRAYFISHES. 337

and fluviatile crabs (Thelphusa) compete for the possession of the freshwaters; and it is not improbable that under some circumstances, they may be more than a match for crayfishes; so that the latter might either be driven out of territory they already occupied, as Astacus leptodactylus is driving out A. nobilis in the Russian rivers; or might be prevented from entering rivers already tenanted by their rivals.

In connection with this speculation, it is worthy of remark that the area occupied by the fluviatile crabs is very nearly the same as that zone of the earth's surface from which crayfish are excluded, or in which they are scanty. That is to say, they are found in the hotter parts of the eastern side of the two Americas, the West Indies, Africa, Madagascar, Southern Italy, Turkey and Greece, Hindostan, Burmah, China, Japan, and the Sandwich Islands. The large-clawed fluviatile prawns are found in the same regions of America, on both east and west coasts, in Africa, Southern Asia, the Moluccas, and the Philippine Islands; while the Atyidæ not only cover the same area, but reach Japan, extend over Polynesia, to the Sandwich Islands, on the north, and New Zealand, on the south, and are found on both shores of the Mediterranean; a blind form (Troglocaris Schmidtii), in the Adelsberg caves, representing the blind Cambarus of the caves of Kentucky.

The hypothesis respccting the origin of crayfishes

which has been tentatively put forward in the preceding pages, involves the assumption that marine Crustacea of the astacine type were in existence during the deposition of the middle tertiary formations, when the great continents began to assume their present shape. That such was the case there can be no doubt, inasmuch as abundant remains of Crustacea of that type occur still earlier in the mesozoic rocks. They prove the existence of ancient crustaceans, from which the crayfishes may have been derived, at that period of the earth's history when the conformation of the land and sea were such as to admit of their entering the regions in which we now find them.

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

FOSSIL ASTACOMORPHA.

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. 10. Carboniferous. 11. Devonian. 13. Cambrian.

Permian.

Silurian.

12.

Now the oldest known member of the group of the