OF DIDELPIIYS VIRGINIAN A. 69 



sation for its loss elsewhere. Rotation of the atlas around the odontoid is also very 

 perfect. Need of fixedness in the cervical portion of the spine results from some of the 

 habits of the animal, which roots, pig-like, in soft earth and decaying vegetation, in search 

 of the worms, etc., that form part of its food. The great spinous processes, as well as 

 the high occipital crest, are required for the attachment of the large muscles by which 

 such actions are rendered easy. The flexion at either end that the column is capable of, 

 undoubtedly has reference to the care of the contents of the pouch. 



Each of the cervical vertebroe has peculiarities whereby it may be recognized ; though 

 there is comparatively little difference between the third, fourth and fifth. The following 

 remarks apply to the conjoined bones as a wdiole, exclusive of the atlas : The neural 

 spines together form a massive arch, culminating at the third bone, rapidly descending in 

 front to the atlas, more graduallj^ sloping behind to the seventh, which has only a I'udi- 

 ment. The middle portion of the arch is the heaviest, widest across, and most closely 

 locked. The neurapoph^^ses are short, thick, heavy and vertical. At the third vertebra 

 they do not project laterally beyond the side of the spines, though they become thinner, 

 longer, more oblique, and spread further apart upon successive vertebrae from before back- 

 ward. They develop large pre- and post-zygapophj'ses, with very oblique articular sur- 

 faces, largely overlapping each other, and firml}'^ interlocking. These processes together 

 form a prominent serrated ridge along the side of the column, separated from the scries of 

 transverse processes by a w^ide and deep sulcus. The " transverse " processes are of great 

 size, and remarkably expanded in their longitudinal axis. Their free terminal edges arc 

 placed a little obliquely, so that the posterior prolongation of each one overlaps the suc- 

 ceeding, and underlies the preceding, by which means the immobility of the column is aug- 

 mented. The seventh alone does not present this longitudinal expansion of its transverse 

 process. The pleurapophyses are confluent throughout the series, producing vertebrarte- 

 rial foramina. Slight hypapophyses are given ofl* by all the vertebrre, appearing as a con- 

 tinuous ridge along the under side of the centrums, which are otherwise broad and flat 

 underneath. A transverse section of a centrum is nearly oval ; of the neural canals some- 

 what horse-shoe-shaped ; or rather stapeform, with its sides a little approaching below, and 

 a slightly convex floor; highest anteriorly, widest posteriorly. The ordinary relative posi- 

 tion of the vertebrae is that which approximates their spines most closely ; hence the 

 seventh, whose spine is deficient, surmounts, rather than directly follows, the sixth, guiding 

 the cord, by a gentle curve, into the dorsal series. Were it not for this, the change in 

 direction, resulting from the difference in the axes of the neck and back in the animal's 

 ordinary attitudes would be at an abrupt angle. The entire series of vertebrae, viewed 

 from the under side, presents a pyramidal figure, whose apex is the odontoid process ; the 

 bodies of the vertebrae, and their transverse processes gradually narrowing from behind 

 forward. 



The atlas (fig. 12) is notable for the great expansion of the di- and neur-apopliyses. 

 The latter form a broad, heavy arch, over the smooth surfiice of which the advancing and 

 overlapping spine of the axis plays freely from side to side. The lateral processes form 

 two large, thin, flat, subcircular plates, moderately pedicellate, reaching on either side far 

 beyond the corresponding processes of other vertebrae, and serving as powerful points 

 d'appui for muscular action. They have no obliquity, in the longitudinal axis, but extend 

 a little behind the posterior margin of the neurapophyses. The so-called "body" 



HEMOIES HOST. 80C. NAT. HIST. VOL. II. 18 



