$38 DISTRIBUTION AND ETIOLOGY OF THE 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 con- 
tinents 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 require- 
ments of the doctrine of evolution. 
Mention has been made of the close affinity between 
the crayfishes and the lobsters—the Astacina and the Ho- 
marina; 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, fol- 
lowed by two pairs of chelate ambulatory limbs, while 
