end-member

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end-member
1. One of two or more simple substances forming the extreme ends of a solid solution series, e.g. albite (NaAlSi3O8) is one end member, and anorthite (CaAl2Si2O8) the other end-member, in the plagioclase series.

2. In palaeontology, one of the two distinct forms between which gradual variation occurs.