A galaxy classification system is proposed in which normal spirals and lenticulars form parallel sequences within which 'early' and 'late' systems are distinguished by means of their disk-to-bulge ratios. A sequence of 'anemic spirals', which occur most frequently in rich clusters, is found to have characteristics that are intermediate between those of vigorous gas-rich normal spirals and gas-poor systems of type S0. The differences between normal spirals (Sa-Sb-Sc), anemic spirals (Aa-Ab-Ac), and lenticulars (S0a-S0b-S0c) are tentatively interpreted in terms of the influence of environment on the evolution of flattened galaxies.
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S. van den Bergh (1976) studied this question.