NO. 3668 PTERODRILUS—HOLT aa 
occupy the same microhabitat. Diatoms make up a goodly part of the 
food of the species of Pterodrilus and they inhabit creeks and branches 
in upland regions, but nothing else is known about their ecological 
requirements. One is forced, then, to discuss their primitive char- 
acteristics and their subsequent specializations as adaptations fitting 
them for unknown ways of life. I shall proceed by describing the 
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orn Yj P, CEDRUS 
sp. DISTICHUS 
\\ / = P. SIMONDSI 
© P. ALCICORNUS 
Ficure 10.—Distribution of certain species of Péerodrilus. 
hypothetical primitive pro-pterodrilus as I conceive it to have been 
and by defending, along the way, the reasons its various character- 
istics must be considered primitive. From these hypothetical con- 
siderations a tentative phylogeny will be derived and this in turn 
will be tested against the distributional data. Thus, a reasonable, if 
not necessarily true, story of the evolution of the genus can be 
written. 
THE PRIMITIVE Preropritus—The ancestors of Pterodrilus 
were the smallest of the North American branchiobdellids, not 
