Hubble sequence
At the time of the initial publication of Hubble's galaxy classification scheme, the existence of lenticular galaxies was purely hypothetical. Hubble believed that they were necessary as an intermediate stage between the highly flattened "ellipticals" and spirals. Later observations (by Hubble himself, among others) showed Hubble's belief to be correct and the S0 class was included in the definitive exposition of the Hubble sequence by Allan Sandage. Missing from the Hubble sequence are the early-type galaxies with intermediate-scale disks, in between the E and S0 type, Martha Liller denoted them ES galaxies in 1966.