334 THE ANATOMY OF VERTEBRATED ANIMALS. 



opens externally, are always situated at the top of tlie head, 

 and far removed from the extremity of the snout. There is 

 no third eyelid, and the very small auditory apertures are to- 

 tally devoid of any pinna. The teats are two, and, in the 

 female, are lodged in depressions on each side of the vulva. 



The articular surfaces of the centra of the vertebrae are 

 flat, and the epiphyses usually remain distinct for a long time. 



The spinal column, as a whole, is remarkable for the short- 

 ness of its cervical, and the length of its lumbar region, there 

 being sometimes a greater number of lumbar than of dorsal 

 vertebras. There is no sacrum. The caudal vertebrae are only 

 distinguishable from the posterior lumbo-sacral vertebrae by 

 their chevron-bones. The second vertebra of the neck is de- 

 void of any odontoid process; and it very commonly happens 

 that more or fewer of the cervical vertebra, the bodies of 

 which are often so short as to be mere disks, are anchylosed 

 together, either by their arches, or by their centra, or by both. 

 The centra of all the succeeding vertebrae are large in propor- 

 tion to their arches, and the inter-vertebral fibro-cartilages are 

 exceedingly thick, so as to confer great flexibility and elasti- 

 city on the spine. The arches of the hinder dorsal vertebrte, 

 and of those of the lumbar and caudal regions, are not articu- 

 lated together by zygapophyses. The centra of the posterior 

 caudal vertebrjB lose their processes and become polj^gonal. 



Very few of the ribs become connected with the sternum 

 at their distal ends; and, in contradistinction to what happens 

 in most 3fammalia, the proximal ends of the majority of the 

 ribs are connected only with the transverse processes of the 

 vertebrae, and not with their bodies. 



The skull is even more remarkably modified than the ver- 

 tebral column. The brain-case itself has a spheroidal form ; 

 while the jaws are greatly prolonged, the principal enlarge- 

 ment of the upper jaw taking place in the region which lies in 

 front of the nasal aperture. The basis crcmii, as a whole, is 

 ramarkably broad, and its upper surface concave from before 

 backward, the sella turcica being very slightly indicated. 

 The parietal bones are comparatively small, and do not meet 

 in a sagittal suture, as they do in other Mamynalia ; the 

 supraoccipital, with an interparietal bone, being interposed be- 

 tween them, and extending forward so as to unite with the 

 frontals. Each frontal bone is produced outward into a great 

 bony plate which covers the orbit. The squamosal bone sends 

 a very large and stout zygomatic process forward to meet this 

 supra-orbital prolongation of the frontal. The proper jugal 



