316 DISTRIBUTION AND A:TIOLOGY 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 eaxcelsus, 
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 Astaci from the freshwater Tertiary of Idaho. Pro- 
ceedings of the American Philosophical Society, 1869-70, 
