332 DISTRIBUTION AND ZTIOLOGY OF THE CRAYFISHES, 
Potamobiide in the northern hemisphere, and another, 
with those of the Parastacida, in the southern hemisphere. 
The ancestral Potamobine form probably presented 
the peculiarities of the Potamobiide in a less marked 
degree than any existing species does. Probably the 
four pleurobranchiz were all equally well developed ; the 
lamine of the podobranchie 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 
Palinuridg, the headquarters of which are in the 
southern hemisphere. That they should have ascended 
