638 



MAMMALIA. 



body, including the posterior clinoid processes, and of the greater 

 alse and pterygoid processes (Jig. 295, 25). The anterior half 

 is formed by the anterior clinoid processes and alse minores 

 (Jig- 295, 11). These two halves may therefore be called, respec- 

 tively, the anterior and posterior sphenoids. 



Lastly, we have the temporal bone, exhibiting but one piece, 

 although made up of all the parts which in the Reptile were so 

 obviously distinct elements. The petrous portion wedged into 

 the base of the cranium, still encloses the internal car. The 

 tympanic element (Jig* 294, a) supports the membrana tympani. 

 The mastoid process (Jig. 295, 12) is the homologue of the mas- 

 toid bone of the Crocodile ; and, lastly, the squamous element 

 with which the lower jaw is articulated (Jig. 294, 23) in the 

 Reptilia, was visibly a distinct bone. Even to these may be added 

 the zygomatic process, which Professor Owen regards as an inde- 

 pendent elemental part. 



(719.) Reviewing, therefore, all that has been said relative to 

 the composition of the skull in the different classes of Vertebrata, 

 the following deductions may be arrived at.* 



1. That, as we advance from lower to higher forms, the propor- 

 tionate size of the cranium relative to that of the face becomes 

 greater. 



2. That the number of bones met with upon the inferior and 

 lateral aspects of the head gradually diminishes : for in Mammalia 

 the pterygoid and tympanic bones which even in Birds are sepa- 

 rate pieces, become very generally confounded with the sphenoid 

 and the temporal ; and also the petrous and squamous portions of 

 the temporal become blended together. 



3. The number of bones normally entering into the composition 

 of the cranium of adult Mammalia varies considerably. When most 

 numerous, there are twenty-eight, eleven in the cranium, and 

 seventeen in the face. In this case the cranial bones are the follow- 

 ing, one occipital, one sphenoid, the two squamous portions of 

 the temporal, the two tympano-petrous portions of the temporal, the 

 two parietal, the two frontal, and the ethmoid. 



The bones of the face are, two superior maxillary, two inter- 

 maxillary, two nasal, two lacrymal, the vomer, two inferior turbi- 

 nated bones, two palate bones, two jugal bones, and, lastly, the two 

 halves of the lower jaw. 



It is true that some slight exceptions occur : thus, for example, 



* Meckel, Trait6 G6nerale d'Anatomie Comparee, torn. iii. seconde partie, p. 195. 



