THE GLOBE PARTITIONED BY CEAYEISHES 209 



]iarity that has long been known in the genus Gambarus. 

 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 Potamobiidas 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. — Parastacidoe. 



These agree very closely with the preceding family 

 except in regard to the branchi^, 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 podobranchi^ of the 

 following appendages are devoid of more than a rudiment 

 of a lamina, while some of their filaments and attendant 

 setae 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 

 livers of Europe and Asia, and five species of that genus 

 exist in rivers of North America, west of the Eoeky 

 Mountains, whereas fifty-two species of Gambarus inhabit 

 the rivers and lakes of North America east of that range. 

 Of the Parastacidse, Astacoides, Gru6rin, 1839, with its 

 solitary species mada/jascwriensis, is found only in Mada- 

 gascar; Parastacus, Huxley, 1878, was established for 



