The primary difference between lenticular and spiral galaxies lies in the presence or absence of spiral arms around the galaxy's central bulge. According to the Sloan Digital Sky Survey, spiral galaxies have two distinct regions, a large central bulge and the outlying spiral arms. Lenticular galaxies also have a central bulge, but their outlying stars are less organized and more diffuse.
Lenticular galaxies have a structure that is in many ways similar to spirals with a flat disk surrounding a central bulge. The similarity breaks down, however, in their lack of organized spiral arms. For this reason, according to the Sloan Digital Sky Survey, lenticular galaxies are sometimes referred to as "armless spirals." This distinct lack of structure makes lenticular galaxies, such as M85, easy to mistake for ellipticals, especially under less-than-optimal conditions such as low light levels. Like some spiral galaxies, some lenticular galaxies are crossed along their plane by a noticeable band of stars and gas. Lenticular galaxies without such a band are technically known as Type S0. Banded lenticular galaxies are designated SB0. Unbanded Type S0 lenticular galaxies can easily be mistaken for Type E0 ellipticals if they are oriented in a way that makes it difficult to perceive their flattened-disk shape.