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Description
After Triceratops was described, between 1889 and 1891 Hatcher with great effort collected another thirty-one of its skulls. The first species had been named Triceratops horridus by Marsh. Its specific name was derived from the Latin horridus; "rough, rugose", perhaps suggesting the roughened texture of those bones belonging to the type specimen, later identified as an aged individual. The additional skulls varied to a lesser or greater degree from the original Triceratops specimen. This variation is unsurprising, given that Triceratops skulls are large three-dimensional objects, coming from individuals of different ages and both sexes, and which were subjected to different amounts and directions of pressure during fossilization.[13]

However, not a single one of these skulls was referred to T. horridus by Marsh who instead named eight further species and eventually even a new genus Sterrholophus. In 1889, he named two species. Triceratops flabellatus, the "fan-shaped", was based on skull YPM 1821. Triceratops galeus, "the helmeted one", was exceptionally based on a specimen not found by Hatcher, USNM 2410, a horn and frill excavated by George Homans Eldridge in Colorado in the Laramie Formation. In December 1889, Marsh published the first illustration ever of a Triceratops skull, that of T. flabellatus. In January 1890, two additional species were added. Triceratops serratus, "the serrated one", was based on skull YPM 1823, and Triceratops prorsus, "pointing forward", on specimen YPM 1822, a nose horn. In May 1890, Triceratops sulcatus was added, named "the one with troughs" because of grooves on the horns, based on the fragmentary skull USNM 4276. On this occasion, Marsh reported a natural brain cast, that he considered remarkably small. In February 1891, Marsh published additional images, for the first time concluding that the animal walked on all fours: initially he had assumed it was a bipedal form, brought to the brink of extinction by its overly heavy skull. A full skeletal restoration followed in April, together with one of Brontosaurus in the same publication, the first for Mesozoic dinosaurs ever drawn. The same year he renamed T. flabellatus into the separate genus Sterrholophus, the "stiffly crested". In September, he named Triceratops elatus, "the raised one", based on specimen YPM 1201, an enormous skull with an upwards pointing low nose horn found on 23 October 1890 by Hatcher. Marsh named his last Triceratops species in 1898, when part of his collection was transferred from Yale to the Smithsonian Institution. Triceratops calicornis, "the chalice horned", was based on specimen USNM 4928, a skull with a strange depression on the rear of the horn base. Triceratops obtusus, the "stumpy one", was based on specimen USNM 4720 with an obtuse nose horn.

After Marsh's death, Hatcher attempted to revise the material but fell ill writing the study, never to recover. Richard Swann Lull completed his monograph. Lull and subsequent researchers would disagree as to the number of separate species (listed below), and came up with several phylogenetic schemes for how they were related to each other.

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.[23] 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.[25] 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.[25][26] 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.[27] This pattern was followed until the major studies of the 1980s and 1990s.

Description
After Triceratops was described, between 1889 and 1891 Hatcher with great effort collected another thirty-one of its skulls. The first species had been named Triceratops horridus by Marsh. Its specific name was derived from the Latin horridus; "rough, rugose", perhaps suggesting the roughened texture of those bones belonging to the type specimen, later identified as an aged individual. The additional skulls varied to a lesser or greater degree from the original Triceratops specimen. This variation is unsurprising, given that Triceratops skulls are large three-dimensional objects, coming from individuals of different ages and both sexes, and which were subjected to different amounts and directions of pressure during fossilization.[13]

However, not a single one of these skulls was referred to T. horridus by Marsh who instead named eight further species and eventually even a new genus Sterrholophus. In 1889, he named two species. Triceratops flabellatus, the "fan-shaped", was based on skull YPM 1821. Triceratops galeus, "the helmeted one", was exceptionally based on a specimen not found by Hatcher, USNM 2410, a horn and frill excavated by George Homans Eldridge in Colorado in the Laramie Formation. In December 1889, Marsh published the first illustration ever of a Triceratops skull, that of T. flabellatus. In January 1890, two additional species were added. Triceratops serratus, "the serrated one", was based on skull YPM 1823, and Triceratops prorsus, "pointing forward", on specimen YPM 1822, a nose horn. In May 1890, Triceratops sulcatus was added, named "the one with troughs" because of grooves on the horns, based on the fragmentary skull USNM 4276. On this occasion, Marsh reported a natural brain cast, that he considered remarkably small. In February 1891, Marsh published additional images, for the first time concluding that the animal walked on all fours: initially he had assumed it was a bipedal form, brought to the brink of extinction by its overly heavy skull. A full skeletal restoration followed in April, together with one of Brontosaurus in the same publication, the first for Mesozoic dinosaurs ever drawn. The same year he renamed T. flabellatus into the separate genus Sterrholophus, the "stiffly crested". In September, he named Triceratops elatus, "the raised one", based on specimen YPM 1201, an enormous skull with an upwards pointing low nose horn found on 23 October 1890 by Hatcher. Marsh named his last Triceratops species in 1898, when part of his collection was transferred from Yale to the Smithsonian Institution. Triceratops calicornis, "the chalice horned", was based on specimen USNM 4928, a skull with a strange depression on the rear of the horn base. Triceratops obtusus, the "stumpy one", was based on specimen USNM 4720 with an obtuse nose horn.

After Marsh's death, Hatcher attempted to revise the material but fell ill writing the study, never to recover. Richard Swann Lull completed his monograph. Lull and subsequent researchers would disagree as to the number of separate species (listed below), and came up with several phylogenetic schemes for how they were related to each other.

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.[23] 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.[25] 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.[25][26] 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.[27] This pattern was followed until the major studies of the 1980s and 1990s.