In an influential 1953 publication, primatologist William Charles Osman Hill also consolidated all the slow lorises in one species, "Nycticebus coucang", and considered other forms distinct at the subspecies level. Osman Hill thus listed "Nycticebus coucang pygmaeus", while acknowledging that "it may be deemed necessary to accede this form specific rank." In 1960, Dao Van Tien reported a species from Hòa Bình Province, Vietnam, that he called "N. intermedius", but it turned out that his specimens were merely adults of the pygmy slow loris, which had originally been described on the basis of a juvenile. After studying slow lorises from Indochina, primatologist Colin Groves proposed that the pygmy slow loris was morphologically unique enough to be considered a distinct species. The validity of this opinion was later corroborated by studies of chromosomal structure, genetic distance determined by protein variation at polymorphic loci, and mitochondrial DNA restriction enzyme analysis.