332 DISTRIBUTION AND .ETIOLOGY OF THE CRAYFISHES. 



Potamobiida in the northern hemisphere, and another, 

 tfith those of the Parastacida, in the southern hemisphere. 



The ancestral Potaniobine form probably presented 

 the peculiarities of the Potamobiida in a less marked 

 degree than any existing species does. Probably the 

 four pleurobranchise were all equally well developed ; the 

 laminaB of the podobranchJse 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- 

 mobine, it must have approached the common form in 

 which Homarus and Nephrops originated. And it is 

 to be remarked that these also are exclusively confined 

 to the northern hemisphere. 



The wide range and close affinity of the genera 

 Astacus and Cambarus appear to me to necessitate the 

 supposition that they are derived from some one already 

 specialised Potamobine form ; and I have already men- 

 tioned the grounds upon which I am disposed to believe 

 that this ancestral Potamobine 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 

 Pal'uiuridte, the headquarters of which are in the 

 southern hemisphere. That they should have ascended 



