162 PROCEEDINGS OF THE NATIONAL MUSEUM. vol. xxxii. 



phj'ses at an ascending angle. The inferior blade of the diapophysial 

 lamina connects the transverse process with the side of the neural 

 arch, thus forming the anterior wall of the postdiapophysial cavity. 

 The pre- and supradiapophysial cavities are not present either in 3L 

 agilis or M. grandis. On the left side, just in front of the pleurocen- 

 tral cavity, is a small foramen (y, Plate XII) which is wanting on the 

 opposite side. The axis of 31. agilis may be distinguished from that 

 of 2f. grandis by the more anteriorly directed spine of the former, by 

 the posterior extension of the postzygapophyses, and by the greater 

 length of the centrum in proportion to its height. The writer believes 

 that the cavities can not be considered sufficiently reliable characters 

 for even specific separation, as they are not at all constant in shape, 

 size, or position. Even on the same vertebra, as shown in this speci- 

 men, cavities found on one side may be absent on the other. 



Measurements. 



mm. 



No. 5384. Greatest length of centrum of axis 70 



No. 5384. Greatest height of axis (taken at the middle ) 97 



Third cervical.— This vertebra was articulated with the axis, so its 

 position may be considered absolutely determined. Like the other 

 elements of the type specimen, it has suffered somewhat from lateral 

 crushing. The transverse processes, postzygapophj'sial lamina, and 

 postzygapophyses are wanting. This is the first vertebra of the ver- 

 tebral column to have the anterior zygapophyses prolonged beyond 

 the end of the ball of the centrum. Although the transverse processes 

 are missing, a well-developed articular facet (see /■, Plate XII) on the 

 lower anterior margin of the centrum shows the place of attachment 

 for the capitulum of the cervical rib. 



The spinous process, 'as figured, has been crushed forward somewhat 

 from its normal position. This spine anteriorly is a thin plate of bone 

 formed by the union of the prezygapophysial lamina, but shows no 

 indication of being bifid at its apex, as Hatcher has represented the 

 spine on the third cervical of Diplodocus., or as the fourth cervical 

 of M. grandis is known to be. The large pleurocentral cavities 

 posteriorly are separated by a thin septum of bone, but anteriorly 

 this partition fails and they ))ecome confluent. As in the. axis, this 

 cavity occupies the posterior half of the centrum. On the anterior 

 part of the centrum just above the facet for the cervical rib is another 

 lateral cavity, nearly round in outline and extending well into the ball 

 of the centrum. There' is also a prediapophysial cavity which is 

 separated from the postdiapophysial cavity by the lower blade of the 

 diapophysial lamina. This cavity is not present on the right side of 



the centrum. 



rriiicipal measuremeuis. 



mm. 



No. 5384. Greatest length of centrum of third cervical 95 



No. 5384. Greatest height of third cervical 3 00 



