Acquaviva (2008:25)

 "In languages with a collective–singulative opposition, the label 'collectives'
 refers to noun forms that are plural by syntactic and morphological criteria,
 but differ from regular count plurals in the conceptualization of their reference
 domain. 
 Loosely speaking, they are true not of sets of distinct individuals
 but of manifold masses, whose elements contrast with whole individuals in
 various possible ways, traceable to lack of unity and/or of identity. 
 Typical examples from Breton are bili 'gravel', gwer 'glasses, glassware', gwez 'trees',
 kelien 'flies', merien 'ants', nez 'lentils', stered 'stars'."
 

Bibliographie

• Acquaviva, P. 2008. Lexical plurals, A morphosemantic Approach, Oxford University Press. [summary]