THE SKULL. 63 



II. Bones of the Outer Surface (enumerated from before 

 backwards). 



1. Premaxilla. 



2. Maxilla. 



3. Nasal. 



4. Lacrymal. 



5. Frontal. 



INVESTING 

 BONES. 



6. Prefrontal (of Keptiles). 



7. Postfrontal or postorbital. 



8. Supraorbital. 



9. Parietal. 



10. Temporal or squamosal. 



11. Supraoccipital (in part). 



III. Cartilage Bones. 



* ^ Present only in Amniota (forming the base of the 



2. Basisphenoid V , m J 



3. Presphenoid J 



4. Exoccipital (supraoccipital, in part). 



5. Pro-, epi-, and opisthotic, also sphenotic and pterotic (in Teleostei), 



(forming the bony auditory capsule). 



*" A V l ( s P uen id, developed in the trabecular region. 



8. Ethmoid, together with the rest of the cartilaginous skeleton of the 



nose (septum, turbinals, &c.). 



9. Quadrate. 



10. Articular. 



11. Visceral skeleton (in part). 



ANATOMY OF THE SKULL. 

 SPECIAL PART. 



A. Fishes. 



The skulls of Fishes vary so greatly in their details that only a 

 general outline can be given here. 



In the suctorial Fishes, or Cyclostomes, the skull is deve- 

 loped essentially in the manner described already for all Verte- 

 brates. Later, however, the form of the skull shows so many 

 peculiarities, probably in consequence of the suctorial (Petromyzon) 

 or parasitic (Myxine) mode of life of these animals, that it becomes 

 quite abnormal. The most important peculiarity is the absence of 

 proper jaws such as those of other Vertebrates ; for this reason 

 these Fishes are called Cyclostomata to distinguish them from 

 the other Vertebrates or Gnathostomata. Their visceral skeleton, 

 consisting of a delicate cartilaginous basketwork, also shows many 

 peculiarities (Fig. 49), such as, for instance, its very superficial 

 position ; we may accordingly speak of these cartilages as " extra- 

 branchials." 



