o. 



32 DISTRIBUTION AND .ETIOLOGY OF THE CRAYFISHES. 



Potamohiiclce in the northern hemisphere, and another, 

 with those of the ParastacidcB, in the southern hemisphere. 



The ancestral Potamohine form prohahly presented 

 the peculiarities of the Potamohiidce in a less marked 

 degree than any existing species does. Probably the 

 four pleurobranchise were all equally well developed ; the 

 laminse of the podobranchjee smaller and less distinct 

 from the stem ; the first and second abdominal appen- 

 dages less specialised ; and the telson less distinctly 

 divided. So far as the type was less specially Pota- 

 mohine, it must have approached the common form in 

 which Homarus and Nephro;ps originated. And it is 

 to be remarked that these also are exclusively confined 

 to the northern hemisphere. 



The wide range and close afiinity of the genera 

 Astacus and Camharus appear to me to necessitate the 

 supposition that they are derived from some one already 

 specialised Potamohine form ; and t have already men- 

 tioned the grounds upon which I am disposed to believe 

 that this ancestral Potamohine existed in the sea which 

 lay north of the miocene continent in the northern 

 hemisphere. 



In the marine primitive crayfishes south of the equator, 

 the branchial apparatus appears to have suffered less 

 modification, while the suppression of the first abdominal 

 appendages, in both sexes, has its analogue among the 

 Pallnurida, the headquarters of which are in the 

 southern hemis^jhere. That they should have ascended 



