382 



ZOOLOGY. 



the palate and two rows of comb-like teeth on the tongue. 

 In the lamprey the edges of the circular mouth are provided 

 with circular rows of conical horny teeth. The teeth of 

 higher Vertebrates are derived from the cells of the mucous 

 membrane of the mouth, which is formed of connective tis- 

 sue as well as epithelium. The teeth of fishes are developed 

 not only in one or several rows in the lip, but may also arm 

 ilie bony projections into the mouth-cavity of the palate, 

 vomcr and parasphenoid bones and the hyoid and bran- 

 chial arches. In the Amphibia teeth survive on the palatine 

 and vomerine bones, more rarely on the parasphenoid ; among 

 the reptiles, the snakes and lizards alone have teeth on the 

 palatine and pterygoid bones, while in the crocodiles and in 

 mammals the teeth are confined to the maxillary bones. In 

 the geckos, snakes and the crocodiles, as well as the mam- 

 mals, the teeth are inserted in sockets (alveoli) of the jaw. 

 (Gegenbaur. ) 



In certain extinct birds (Odontornithes) there were teeth 

 in the jaws, though all existing birds are toothless. It is 



said that rudimentary 

 teeth were found by 

 Geoffrey St. Hilaire in 

 the jaws of a parrot. 

 Blanchard afterwards 

 found the germs of 

 tooth there, though 

 they never come 

 through. In the Mam- 

 mals the teeth are dif- 

 ferentiated into inci- 

 sors, canines, premo- 

 lars and molars (Fig. 

 383). In descriptive anatomy the teeth are for convenience 

 expressed bv a formula, the number of teeth of the upper 

 jaws being placed like the numerator of a fraction, and those 

 of the lower jaw like the denominator, the initials of the 

 names of the teeth being placed before the figures, thus 



Fig. 383. Tooth of tho Tasmanian devil. The 

 incisors are situated in front of the large conical 

 canine trrt'.i. 2, 3, premolars ; m, 1-4, four molar 

 teeth. After Owen. 



the dental formula of man is 





, C 



2-2 1-1 



P d?, J/ 



2-2 3-3 



