DIGESTIVE SYSTEM 737 



titions ; and lastly three molars quite like the pre-molars in form, with 

 several roots, but appearing only in the permanent dentition. The num- 

 ber and kind of teeth is expressed by what is known as a dental formula. 

 As already stated, the number and kinds of teeth in the two halves of the 

 jaw are the same, so only one side need be represented in the formula, 

 but, as in some animals the upper and lower jaws do not have the same 

 types and forms of teeth, the formula must take both upper and lower 

 jaws into consideration. The upper figures, therefore, represent one-half 

 the upper jaw and the lower figures one-half the lower jaw. 



DENTAL FORMULA 



i-f, c-K pm-f , m-f 



This is the dental formula for man ; that for the opossum being : 



i-f-, c 



EPIDERMAL TEETH 



Epidermal teeth occur in cyclostomes (Fig. 422, 6) and various lar- 

 val stages of amphibia and monotremes. In the cyclostome these are 

 little cone-like projections of cornified epithelium with an underlying 

 core of integument. These epidermal teeth are differently arranged in 

 the lampreys and myxinoids. "In the latter they are few, there being 

 a single tooth on the palate and two chevron-shaped rows on the top. 

 In the lampreys, nearly the whole inner surface of the oral hood is lined 

 with these teeth of varying shape and there are a varying number upon 

 the tongue. These teeth are used as a means of fastening the animals to 

 their prey and those of the myxinoid tongue are used for boring into the 

 fishes on which those animals feed. In the larval anura (Fig. 318), the 

 edges of the jaws are armed with cornified papillae serving as teeth, the 

 arrangement of which varies in different genera. They are frequently 

 aggregated in dental plates used in scraping the algae from submerged 

 objects. They are not related to the teeth of cyclostomes." 



Baleen or whalebone should be mentioned here. This is formed in 

 large plates of horny material attached to the margins of the upper jaw 

 (Fig. 392). The fringed ends and edges of these plates serve as strainers 

 to extract the food products from the various materials taken in with the 

 water. 



In the embryos of certain lizards and snakes there is a median tooth 

 which projects from the mouth and which is used to rupture the egg cell 

 when the young is ready to escape. Such a tooth is called an egg-tooth. 

 An egg-tooth is formed in the turtles, Sphenodon, crocodiles, birds, and 

 monotromes, but in these cases it is only a thick (sometimes calcified) 

 portion of the epidermis. 



