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 

 I)leura 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 certainl}'' 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. Eecent and 

 Quaternary. 2. Newer Tertiary (Pliocene and Miocene). 

 3. 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 



