CHAPTER XII. 



THE SKULL IN THE CETACEA AND THE SIRENIA. 



Order CETACEA. The animals of this order exhibit some 

 remarkable modifications in the characters of the skull. 



I will first select for description that of a young example 

 of one of the Odontoceti or Toothed Whales, the common 

 round-headed Dolphin or Globictphalus of our coasts. Fig. 

 65 represents a vertical median section of this skull. It 

 will be seen that the cerebral cavity is of a very unusual 

 shape, being short and broad, but extremely high and con- 

 tracted above in fact, somewhat in the form of a truncated 

 cone, with rounded edges. The bones of the basicranial 

 axis are curved upwards at each extremity. They consist 

 of the ankylosed basioccipital (BO) and basisphenoid (BS) 

 separated by a vertical fissure from the presphenoid (PS) 

 and mesethmoid (M), which are also ankylosed, though 

 their original line of separation can still be traced. The 

 pituitary fossa scarcely forms a distinct concavity, and the 

 clinoid processes are almost obsolete. The mesethmoid is 

 very large, and consists of (i) a high and broad vertical 

 plate, which closes in the vacuity between the frontals in 

 the anterior part of the cerebral cavity, and corresponds to 



