296 CLASSIFICATION OF VERTEBRATES. 



amphibia ; thus the facialis is distinct from the trigeminal ; the 

 eye-muscle nerves have distinct roots ; the glossopharyngeal is 

 distinct from the vagus ; the accessorius is a distinct nerve, 

 except in ophidia, and the hypoglossal becomes a cranial nerve, 

 passing through a foramen in the cranial wall. 



The nostrils are usually terminal, but are just in front of the 

 orbits in ichthyosaurs and plesiosaurs. In the lizards the nasal 

 passage is divided into an anterior vestibule and a posterior 

 olfactory region, and in these, as in the ophidia, the amount of 

 olfactory surface is increased by the presence of a turbinal bone. 

 In the turtles, and still more in the crocodiles, the nasal pas- 

 sage is divided horizontally into an upper olfactory and a lower 

 respiratory tract. Glands occur in connection with the nose 

 in most reptiles, while in all except crocodiles and turtles an 

 organ of Jacobson occurs. 



The eyeball is nearly spherical ; the sclerotic which sur- 

 rounds it is cartilaginous, and in it are frequently developed (as 

 in many birds) a ring of bony sclerotic plates. A tapetum is 

 developed in the lacertilia, but the argentea, so characteristic of 

 lower vertebrates, is lacking. In many there is an internal 

 structure, the pecten, homologous with the process falciformis 

 of the fishes. Eyelids are usually present, the third being fre- 

 quently developed. In snakes and some lizards the lids are 

 transparent, and their edges are united together so that a lachry- 

 mal space is enclosed between them and the conjunctiva. In 

 many lizards and in Hatteria the parietal eye (Figs. 90 and 92) 

 is extremely well developed, and is situated in a foramen in the 

 roof of the skull. Many fossil reptiles belonging to different 

 orders have a similar parietal foramen, thus suggesting the for- 

 mer presence of a visual organ in these forms. 



In the inner ear the lagena is large, and in the crocodiles 

 shows the beginnings of a spiral coiling, recalling the cochlea of 

 the mammals. With its increase in length the macula lagenae 

 is correspondingly elongated. The middle ear and Eustachian 

 tube are lacking in adult snakes and amphisbaenans, the colu- 

 mella auris in these forms being embedded in the flesh. The 

 stapes is continuous with the columella, and in rhynchocephalia 

 the columella is connected with the hyoid. In many lizards and 



