THE GLOBE PARTITIONED BY CRAYFISHES 209 
liarity that has long been known in the genus Cambarus. 
It was at one time supposed that one of the forms, not 
much differentiated from the female, might be sterile, and 
that the more highly developed and specialised form was 
the fertile male. But Mr. Faxon, having kept some 
specimens of the latter form under observation, found that 
after pairing at the next exuviation they assumed the less 
differentiated form, and his inference has been generally 
accepted that the two forms alternate in the same individual 
during a certain part of its life. As it is not probable 
that the Potamobiidee have a monopoly of this curious 
changefulness, the chance of its occurrence is one more 
pitfall to be guarded against in the institution of new 
species. 
Family 4.—Parastacidee. 
These agree very closely with the preceding family 
except in regard to the branchize, appendages of the pleon, 
and the telson. Here the first maxillipeds have the 
epipod almost always provided with a certain number of 
well-developed branchial filaments; the podobranchiz of the 
following appendages are devoid of more than a rudiment 
of a lamina, while some of their filaments and attendant 
sete: terminate in hooks. The first segment of the pleon 
has no appendages in either sex, and the appendages of 
the four following segments are large. The telson is 
never divided by a transverse hinge. 
To this family there are allotted six genera, all be- 
longing to the Southern hemisphere, and living, like those 
of the preceding family, only in fresh or brackish waters. 
The facts of distribution in regard to the two families are 
remarkable. Several species of Potamobia are found in 
rivers of Europe and Asia, and five species of that genus 
exist in rivers of North America, west of the Rocky 
Mountains, whereas fifty-two species of Cambarus inhabit 
the rivers and lakes of North America east of that range. 
Of the Parastacidee,. Astacoides, Guérin, 1859, with its 
solitary species madagascariensis, is found only in Mada- 
gascar; Purastacus, Huxley, 1878, was established for 
