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University of Michigan 



n:ents are given in the accompanying table. Besides the us- 

 ually more depressed form, P. dunkeri differs from P. quit en- 

 sis ( Pfr.) in color (see key). In addition, the growth lines 

 of the latter are developed into plications in the umbilical re- 

 gion. However, P. gigantea ("Gray" Sowerby) has both red- 

 dish and green color-forms, and P. gigantea Hscheri (Hidalgo) 

 jhows the umbilical prominence of the growth lines. In real- 

 ity, no very distinct characters, for the specific separation of 

 many of the forms in the group of P. inca (Orbigny), have 

 been found as vet. 



