Genus Tyrannosaurus Osborn 1905, p. 259

Type species: Tyrannosaurus rex

Diagnosis (See Osborn 1906, 1912, 1917; Matthew and Brown 1923)

Premaxilla contacts nasal below external naris. Maxillary teeth relatively larger than in Daspletosaurus. Anteriormost antorbital fenestra within maxilla not visible in lateral aspect, first antorbital fenestra slightly higher than long. Nasals narrowly constricted between lacrimals. Frontal essentially limited to anteromedial wall of supratemporal fenestra, only small rim of bone reaches dorsal surface of skull. Prefrontal exposed dorsally between lacrimal and frontal. Lacrimal ' horn' represented by conical inflation not interrupting dorsal profile of skull. Postorbital ' horn' or rugose area well developed. Anteroexternal edge of antorbital ramus of lacrimal not continued across lateral surface of jugal. Ventral process of ectopterygoid inflated with very large, ventrally opening sinus. Angular terminates posteriorly behind surangular foramen.

Ventral openings of main basisphenoid sinus situated on either side of midline of skull, dorsomedial to each basipterygoid process. Vertical plate of presphenoid contacts orbitosphenoid and narrow suboptic bridge of laterosphenoid, separating optic fenestra into two openings. Cranial nerves III and IV open into single pit ventrolateral to optic fenestra. Exit for cranial nerve Vi situated close to anterolateral edge of laterosphenoid, but not so close as in Albertosaurus.

Neural spines of fourth and fifth cervical vertebrae subequal, rather narrow in lateral aspect. Neural spine of fifth cervical distally pointed, that of sixth vertebra smaller and sharply pointed. In adults length of humerus equal to about 28 per cent of that of femur. Anterior blade of ilium does not cover diapophysis of twelfth thoracic vertebra. In adults circumference of femur approximately equal to 41 per cent of length of femur. Bodily proportions generally as in Daspletosaurus, although adult specimens about 25 per cent larger, humerus more reduced and ilium longer than in this genus.