THE SKELETON OF THE PORPOISE. 403 



In the spongy texture of all the bones, the absence of 

 medullary cavities in those of the limbs, and in the long 

 persistent separability of the epiphyses of the centra of the 

 vertebrae, the Porpoise resembles other Cetacea ; as it does 

 in the shortness of the cervical, and the length of the 

 lumbar, region of the spinal column. 



The seven cervical vertebrae are all ankylosed together, 

 and the atlas, which is very large in proportion to the rest, 

 overlaps them above and at the sides. The centra of the 

 hinder cervicals are so short and broad that they are mere 

 plates of bone. There are twenty-eight dorso-lumbar verte- 

 brae, of which fifteen are dorsal. In all but the most anterior 

 of these vertebrae, the zygapophyses are abortive ; and long 

 accessory processes, developed from the front part of the 

 neural arches, loosely embrace the spine of the vertebra in 

 front. This arrangement, together with the thickness of 

 the intervertebral ligaments, gives great flexibility to the 

 spinal column. The transverse processes of the hinder 

 dorsal, and of the lumbar, vertebrae are very long. There 

 are five pairs of tme ribs. The sternebras ankylose into 

 an elongated sternum. The anterior caudal vertebrae are 

 provided with large chevi'on bones, and their transverse 

 processes exhibit notches through which branches of the 

 aorta pass. 



In consequence of the globular form of the brain-case, 

 and the prolongation of the jaws, the skiill has a flask-like 

 shape. There is a slight want of symmetry about the 

 base of the upper jaw, but it is hardly appreciable. 



In a longitudinal section, the flatness and the upwardly 

 concave contour of the base of the skull; the extreme 

 shallowness of the sella hircica ; the presence of an ossified 

 tentorium ; and the broad imperforate anterior wall, in the 

 place of the cribidform plate of the ethmoid, are striking 

 features. The synchondrosis between the basi- and pre- 

 sphenoid is persistent. On the base of the skull the basi- 

 occipital gives off great processes oiitwards and downwards, 

 to form, together with a paramastoid prolongation of the 

 exoccipital, and the squamosal, a chamber in which the 



