298 PROCEEDINGS OF THE NATIONAL MUSEUM vf.i,. ii4 



causing older forms of the section to become extinct or isolated in 

 less competitive situations (such as the semilowland habitat occupied 

 by C. virginica). 



4. The preceding events probably occurred prior to and during the 

 Miocene. We can postulate that two major groups of species existed: 

 a primitive group of species in the midcontinental region and a spe- 

 cialized, more rapidly evolving one in the still rugged Appalachians. 

 In late Miocene, however, the moderation of earlier climate brought 

 a change to North America, replacing the subtropical plant life with 

 a more boreal Arcto-Tertiary flora and inaugurating more pronounced 

 seasonal cycles. This now widespread environment may have 

 favored the expansion of specialized cool-water forms westward into 

 the range of older species and must have caused a decline in the 

 abundance of the latter. Possibly the philadelphica stock spread as 

 far west as Colorado at this time. 



5. During the Pliocene abrupt climatic changes occurred, owing 

 to considerable orogeny in the Pacific Coast region and elsewhere, 

 converting the Interior of the continent into a subarid grassland and 

 creating extensive desert areas in the intermontane basins. The 

 distribution of Camharincola was at this time severely fractured, with 

 the Mesochorea section remaining as a rather relict group on both sides 

 of the Continental Divide. The Philadelphica section, which had 

 spread westward only as far as the present site of the Rockies, was 

 thus contained in eastern North America. 



6. The effects of Pleistocene glaciation on Camharincola have 

 probably been only to pre-empt species from higher latitudes in eastern 

 North America. Only fairl}'' widespread and successful species have 

 invaded glaciated terrain. No new species of this genus seem to have 

 evolved during this process. Pterodrilus distichus Moore is known to 

 be virtually restricted to glaciated areas, and three or four species in 

 more highly specialized genera, as yet undescribed, are either endemic 

 to glaciated areas or are most abundant there. Whether this reflects 

 accelerated evolutionary rates or greater adaptability in the occupa- 

 tion of new terrain than in Camharincola I cannot guess. 



It is finally interesting to observe that utilization of crayfish as 

 habitat by branchiobdellids must antedate the evolution of the 

 American cambarine crayfish, since several congeneric species in at 

 least three branchiobdeUid genera inhabit both Pacifastacus of the 

 Astacinae and various genera of the Cambarinae. Without direct 

 geological evidence, we can safely grant the genus Camharincola a 

 considerable antiquity. 



