OSTEOLOGY OF CARNIVOROUS DINOSAURS. 95 



which articulates with the upper concave surface of the intercentrum. The greatest 

 length of the intercentrum is 37 mm.; the greatest transverse width is 54 mm. 



Axis (ax.). — The axis of Ceratosaurus is so fully illustrated in all aspects in 

 plate 19 as to make it unnecessary to enter into a detailed description of it here. 

 This bone may be distinguished at once from the axis of Antrodemus by the well 

 developed longitudinal keel on the ventral surface of the centrum; the better defined 

 and more ventral position of the parapophysis; the more plate-like shape of 

 the neural spine; the absence of notches between spine and. postzygapophyses 

 and the oblique posterior end of the centrum. The second hypocentrum and 

 the odontoid are so thoroughly fused with the centrum that all traces of the 

 sutures have been entirely obliterated. 



The axes of both Ceratosaurus and Antrodemus are to be distinguished from the 

 homologous element in Tyrannosaurus by the relatively longer centra and the more 

 plate-like development of the neural spines. 



Measurements of axis of Ceratosaurus nasicornis Marsh. v 



/ mm. 



Greatest length of centrum+hypocentrum 88 



Greatest width of anterior extremity 53 



Greatest width of posterior extremity 44 



Greatest height over all 117 



Least width of centrum 25 



Cervicals posterior to the axis. — The total number of cervical vertebrae in Cera- 

 tosaurus is somewhat uncertain though it appears to be nine the same as in Antro- 

 demus. The five vertebrae suceeding the axis in the type specimen are in a fair 

 state of preservation. 



The eighth, however, now consists of the neural spine, anterior zygapophyses 

 and. a small portion of the centrum; the ninth by only a median section of the neural 

 spine, as shown in plate 30. Excepting the atlas and axis, which had been entirely 

 freed from the matrix, the remaining cervicals, when they came into my hands, 

 were retained in the rock in their relative positions as shown in the skeleton. All of 

 the centra are strongly cupped on the posterior end, and as Marsh 1 first pointed out, 

 "in place of an equally developed ball on the anterior end there is a perfectly flat 

 surface. The size of the latter is such that it can be inserted only a short distance 

 in the adjoining cup, and this distance is accurately marked on the centrum by a 

 narrow articular border, just back of the flat anterior face. This peculiar articula- 

 tion leaves more than three-fourths of the •cup unoccupied by the succeeding verte- 

 bra, forming apparently a weak joint." In Antrodemus there is a well-developed 

 ball at the cup preceding it, and a flattening of this ball is only to be observed in the 

 transitional vertebrae at the base of the neck. 



The third cervical, which may be considered typical of the anterior part of the 

 series, is shown in all aspects in plate 20, figures 1 to 4. It differs principally from 

 the third of Antrodemus by the presence of a more decided ventral keel. The top of 

 the neural spine is capped with an irregular shaped bony ossification evidently of 

 dermal origin. 



1 Dinosaurs of North America, 1896, pp. 159, 160. 



