In the first attempt to understand the many species, Lull found two groups, although he did not say how he distinguished them: one composed of "T. horridus", "T. prorsus", and "T. brevicornus"; the other of "T. elatus" and "T. calicornis". Two species ("T. serratus" and "T. flabellatus") stood apart from these groups. By 1933, and his revision of the landmark 1907 Hatcher-Marsh-Lull monograph of all known ceratopsians, he retained his two groups and two unaffiliated species, with a third lineage of "T. obtusus" and "T. hatcheri" that was characterized by a very small nasal horn. "T. horridus"-"T. prorsus"-"T. brevicornus" was now thought to be the most conservative lineage, with an increase in skull size and a decrease in nasal horn size, and "T. elatus"-"T. calicornis" was defined by large brow horns and small nasal horn. C. M. Sternberg made one modification, adding "T. eurycephalus" and suggesting that it linked the second and third lineages closer together than they were to the "T. horridus" lineage. This pattern was followed until the major studies of the 1980s and 1990s.