48 



THE CERATOPSIA. 



anterior cervical region and better support to the enormous head the 3 [4] anterior cervicals 

 are fused together by their centra, neural arches, and spines. The cup on the anterior 

 extremity of the atlas for the reception of the occipital condyle is deep and circular. In the 

 type of Triceratops prorsus (No. 1822, Yale Museum) it has a depth of 35 millimeters and a 

 diameter of 87 millimeters. The atlas [axis] bears no cervical rib, but midway between its 

 anterior and posterior extremities on either side on its superior border it sends upward, back- 

 ward, and inward a broad, thin process. These converge and unite medially with the neural 

 spine, forming the anterior border of the neural canal and arching over superiorly the elongated 

 vertebrarterial canals. The anterior opening of the neural canal is large and triangular in 

 outline with the apex of the triangle at the top. Posterior to the vertebrarterial canal the 

 neural arch and spine are completely fused with those of the axis [third cervical], leaving no 

 trace of the anterior zygapophyses of the axis [third cervical] or the posterior zygapophyses of 

 the atlas [axis]. 



The neural spine of the axis [third cervical] is much compressed anteriorly, where, with that 

 of the atlas [axis], it forms a sharp ridge, sloping gently backward and upward. Posteriorly it is 

 higher, and in the type of Triceratops prorsus (No. 1822, Yale Museum) somewhat flattened 

 antero-posteriorly and expanded transversely, while in the type of Triceratops orevicornus (No. 

 1834, Yale Museum) the neural spine of the axis [third cervical] continues compressed to the very 



summit. In the types of both spe- 

 ^J/i.,- cies the posterior zygapophyses of 



the axis [third cervical] are coossi- 

 fied with the anterior zygapophy- 

 ses of the third [fourth] cervical. 

 Beneath and in front of the coossi- 

 fied zygapophyses a large circular 

 foramen passes quite through from 

 one side to the other and affords a 

 means of exit for the spinal nerves 

 given off between these vertebra 5 . 

 Above and posterior to the zyga- 

 pophyses the neural spine of the 

 axis [third cervical] is closely applied 

 to though not coossified with that of the third [fourth] cervical. Just at the middle of the 

 base of the neural arch of the axis [third cervical] there is on either side a prominent diapophy- 

 sis, and some distance below and a little anterior to this a strong parapophysial process springs 

 from the lateral border of the centrum at a point about midway between its superior and 

 inferior borders. These processes, which together represent the superior and inferior branches 

 of the transverse processes, give support to the first pair of cervical ribs. They are separated 

 by a rather deep, long cavity in the external border of the centrum. 



The centrum of the third [fourth] cervical is a little shorter than that of either the atlas 

 [atlas and axis] or axis [third cervical], and it. is firmly .coossified with that of the latter. The 

 neural arches of these two vertebrae are likewise firmly coossified, as are also their opposing 

 zygapophyses. Above the zygapophyses the spines are free, though closely applied to one 

 another. The neural spine of the third [fourth] cervical is a little higher than that of the 

 axis [third cervical] and has the summit much expanded transversely and slightly emarginate. 

 The diapophysis occupies a rather more elevated position than in the axis [third cervical]. 

 It is both longer and stronger than in that vertebra. It is directed forward and arches down- 

 ward and bears at its extremity an articular facet for the tuberculum of the second cervical 

 rib. At its base it is separated from the parapophysis by a concave surface which is not so 

 deep as the corresponding one on the axis [third cervical]. The parapophysis is placed rather 

 more anterior than that of the axis [third cervical] and is not so prominent, being reduced 

 to a small round articular surface on the anterior portion of the body of the centrum. From 



Pig. 50. — A, Anterior cervicals of type of Triceratops prorsus. No. 1822, Yale 

 Museum; B, posterior view of fourth [fifth] cervical of same, a, Anterior face 

 of atlas; d, diapophysis; n, neural canal; p, posterior face of fourth [fifth] ver- 

 tebra; r, rib; s, neural spine of axis; s'. neural spine of third [fourth] cervical; 

 s", neural spine of fourth [fifth] cervical; z, posterior zygapophyses. One-eighth 

 natural size. After Marsh. 



