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 Homa- 
rina 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). 
8. Older Tertiary (Eocene). 4. Cretaceous (Chalk, 
Greensand and Gault). 5. Wealden. 6. Jurassic (Pur- 
beck to Inferior Oolite). 7. Liassic. 8. Triassic. 9. 
Permian. 10. Carboniferous. 11. Devonian. 12. 
Silurian. 13. Cambrian. 
Now the oldest known member of the group of the 
z2 
