1974] 
Coyle — Genus Aliatypus 
435 
Map 1. Known distribution ranges of Aliatypus species in relation to 
present major arid habitat barriers which formed during the retreat of 
the Wisconsin glaciation. 
diaetid trans-valley connections existed during the most recent (Wis- 
consin) glacial period and consequently became severed as recently 
as 13,000 years ago. Perhaps the A. calif ornicus population at Mari- 
posa, which is phenotypically more distinct from the coastal popula- 
tion than is the north Sierran population, was last connected with 
the coastal population during an earlier glacial period ; or perhaps its 
trans-valley connections were simply severed earlier during the retreat 
of the last (Wisconsin) glacial period than were those of the north 
Sierran populations. Perhaps continued expansion of habitat barriers 
during the present post-glacial period has tended to restrict gene flow 
between the morphologically divergent northern populations of A. 
erebus and its south Sierran populations. 
There is much geographic variation in Aliatypus janus, but the 
samples are so small and scattered that it is difficult to recognize 
important barriers to gene flow. The northernmost samples probably 
