316 DISTRIBUTION AND ^ETIOLOGY OF THE CRAYFISHES. 



do ; but I am not aware that any member of the group is 

 found in Madagascar, and thus completes the analogy. 



The preservation of the soft parts of animals in the 

 fossil state depends upon favourable conditions of rare 

 occurrence ; and, in the case of the Crustacea, it is not 

 often that one can hope to meet with such small hard 

 parts as the abdominal members, in a good state of 

 preservation. But without recourse to the branchial 

 apparatus, and to the abdominal appendages, it might be 

 very difficult to say whether a given crustacean belonged 

 to the Astacine, or to the closely allied Homarine group. 

 Of course, if the accompanying fossils indicated that the 

 deposit in which the remains occur, .was of freshwater 

 origin, the presumption in favour of their Astacine nature 

 would be very strong ; but if they were inhabitants of the 

 sea, the problem whether the crustacean in question was 

 a marine Astacine, or a true Homarine, might be very 

 hard to solve. 



Undoubted remains of crayfishes have hitherto been 

 discovered only in freshwater strata of late tertiary age; 

 In Idaho, North America, Professor Cope * found, in 

 association with Mastodon mirificus, and Equus excelsus, 

 several species, which he considers to be distinct from 



the western side of the Rocky Mountains are different from the Eastern 

 American forms, yet there are species common to both the Asiatic and 

 the American coasts of the North Pacific. 



* On three extinct Agtaei from the freshwater Tertiary of Idaho. Pro- 

 ceedings of the American Philosophical Society, 1869-70. 



