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 Globicephalus of our coasts. Fig. 

 6 1 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 contracted 

 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) sepa- 

 rated by a vertical fissure from the presphenoid (BS) and 

 mesethmoid (ME), 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 the 

 cribriform plate of the ordinary Mammal, though with but few 



Id small perforations ; (2) an anterior rod-like, somewhat 



