THE SKELETON OF THE PORPOISE. 343 



the anus in the male ; while, in the female, the interval, in 

 which the fossse which lodge the teats are situated, is much 

 less. There is a conspicuous vertical dorsal fin in addition to 

 the flattened caudal fin. Immediately beneath the skin is a 

 thick layer of blubber, as in other Cetacea. 



In the sjDongy texture of all the bones, the absence of me- 

 dullary cavities in those of the limbs, and in the long per- 

 sistent separability of the epiphyses of the centra of the verte- 

 brae, 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 vertebn^ are all anchylosed together, and 

 the atlas, which is very large in proj:>ortion to the rest, over- 

 laps 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 twentj'-'eight dorso-lumbar vertebras, 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 vertebr.'B in front. This ar- 

 rano-ement, to2:ether 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 true ribs. The 

 sternebrae anchylose into an elongated sternum. The anterior 

 caudal vertebrae are provided with large chevron-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 skull 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 shallow- 

 ness of the sella turcica / the presence of an ossified tentorium ; 

 and the broad imperforate anterior wall, in the place of the 

 (iribriform plate of the ethmoid, are striking features. The 

 synchondrosis between the basi- and presphenoid is persistent. 

 On the base of the skull the basi-occipital gives off great pro- 

 cesses outward and downward, to form, together wdth a para- 

 mastoid prolongation of the exoccipital, and the squamosal, a 

 chamber in which the anchylosed tympanic and periotic bones 

 are contained. The ex- and supra-occipitals, together with the 

 interparietals, form the whole back wall and middle of the 



