n6 



LABORATORY MANUAL FOR VERTEBRATE ANATOMY 



median ventral plate, the body of the hyoid, located in the floor of the mouth; 

 it represents the ventral ends of the hyoid and other gill arches. From it pro- 

 ject posteriorly two pairs of slender processes, known as the anterior and posterior 

 horns of the hyoid. These are portions of the third and fourth gill arches. 

 10. The teeth.— The teeth of the alligator occur in a single row on the 

 premaxillae, maxillae, and dentary bones. Their structure is the same as that 

 already described under the exoskeleton. The pulp cavity of these teeth is 

 widely open at the base, and the teeth can therefore be replaced an indefinite 

 number of times, each successive tooth having the same pulp cavity as its 



parietal 



alisphenoid 



squamosal 



orbitosphenoid 

 presphenoid 



nasal 

 ectethmoid 

 lacrimal 



mesethmoid 

 vomer 

 premaxilla 



supraoccipital 



petromastoid 

 ;"■ :\, stapes (quadrate) 



exoccipital 



basioccipital 



incus (hyomandibular 

 malleus (articular) 



Meckel's cartilage 

 pterygoid 



dentary 



palatine 



Fig. 39. — Diagram of the bones of the mammalian skull. Membrane bones blank; cartilage 

 bones stippled. (After Kingsley's Comparative Anatomy of Vertebrates, copyright by P. Blakiston's 

 Son and Company.) 



predecessor. The teeth are set into sockets or alveoli in the jaw; teeth so placed 

 are said to be thecodont. The teeth are all alike in size and form; hence they are 

 said to be homodont. 



G. THE BONES OF THE MAMMALIAN SKULL 



The mammalian skull is completely ossified with the exception of a small 

 part of the ethmoid region. The number of bones which it contains is, however, 

 considerably less than that found in reptile skulls. This is due in part to a loss 

 of bones and in part to a fusion of persisting bones. This fusion is most marked 

 in the human skull. Thus, as Williston says, ll The most primitive reptiles had 

 no less than seventy-two separate bones in the skull; the human skull has but 

 twenty-eight inclusive of the (six) ear bones." The accompanying diagram oi 

 the mammalian skull, Figure 39, should be studied carefully and will aid in 

 acquiring an understanding of the construction of the mammalian skull. The 



