THE DIGESTIVE AND RESPIRATORY SYSTEM 277 



:he last figure representing the total number of cheek teeth, 

 without attempting a distinction between premolars and mo- 

 lars. Among placental mammals the Insectivora show the 

 most primitive dentition; for example, that of Talpa (the 

 mole) is: 



3-1-4-3 



3-1-4-3 



Certain cases, on the other hand, show an extraordinary re- 

 duction, as in the mouse-like rodent, Hydromys, where the 

 formula is: 



The greatest reduction is found among some Cetacea, and in 

 certain edentates (Myrmecophaga). In certain of the first 

 group the tooth germs never develop but remain within the 

 gums, while feeding is carried on through the development of 

 horny fringes (the so-called " whale-bone ") depending from 

 the upper jaw, and forming a filter, which transforms the 

 mouth into a scoop-net for the apprehension of shoals of 

 marine creatures of various sorts. In the duck whale, Hy- 

 peroodon, there are but four teeth, all in the lower jaw, the 

 anterior ones of conical shape, behind which are two imbed- 

 ded in the gums ; in the narwhal, Monodon, there are numer- 

 ous small teeth that fall out before maturity, leaving the jaw 

 toothless, but in the male a single upper tooth, usually that 

 of the left side, develops into a long, projecting horn so large 

 that it renders the skull asymmetrical. In certain of the 

 smaller Cetacea, dolphins and porpoises, the jaws are fur- 

 nished with conical teeth, nearly or quite homodont, which 

 often surpass in number those of any other mammals (in the 

 Dolphin, Delphinus, 47-65 in each half-jaw). Both this ex- 

 cessive number, which is at variance with the general formula 

 for mammalian dentition, and their homodont character, must 

 be looked upon as secondary adaptations to aquatic conditions. 

 In the primates there are two incisors upon each side and 

 a well-developed canine. In the monkeys of the Western 



