338 DISTRIBUTION AND JSTIOLOGY OF THE CEAyFISHES. 



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 eai'th'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 aU 

 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 



