36 MAMMALIA. 



Hystrix. 3d, Compound teeth, denies compositi, in which each molar 

 tooth consists of separate laminae covered with enamel, and united 

 by means of a softer intervening substance, called crusta, petrosa, or 

 cementuin. This structure is most clearly and strikingly displayed 

 in the large molars of the elephant, but it occurs also in the Horse, 

 the Ruminantia, and many Rodentia, as the Hare, the Field-mouse 

 (Arvicola), and the Guinea-pig. 



The diversities in the form and arrangement of the teeth are so 

 great, that scarcely anything general can be said about them. It 

 is the special province of zoology to set forth these specialities, and 

 we shall therefore here give only a few of the prominent examples. 

 Thus in the Narwhal a very peculiar formation and asymmetrical 

 arrangement of the teeth occurs. There is usually found only upon 

 one side of the upper jaw a very long spear-shaped projecting tusk, 

 while that of the other side remains quite rudimentary, and is prob- 

 ably a mere deciduous tooth ; the rest of the teeth are want- 

 ing. Hyperoodon has only some small teeth in the lower jaw. The 

 Dolphins have a great many, often 200, mostly pointed teeth, in 

 both jaws. The graminivorous Cetacea, Halicore and Manatus, have 

 merely molar teeth with flat crowns ; in the first the incisor teeth 

 in the upper jaw are developed into long deflexed tusks. In the 

 Ruminantia generally the superior incisor teeth are wanting in the 

 intermaxillary bone, which in the Camels only supports a pair of 

 incisors resembling canines. The canine teeth, with the exception 

 of the Musk-deer and the Camel, are also wanting in the Rumi- 

 nantia. In the Horse the males only have canines, but here also 

 they are frequently undeveloped. Among the Pachydermata the 

 canines are wanting in the Rhinoceros, in the Hyrax, which has 

 rodent-shaped incisors, and in the Elephant, in which incisor teeth 

 are also wanting in the lower jaw, while those in the intermaxillary 

 project as long tusks. The Ornithorynchus has, upon the whole, 

 above and below, four singular horny molar teeth. The incisors are 

 wanting in all the Edentata, Dasypus sexcinctus only having two 

 upper ones ; the canines also are wanting in nearly all, and the 

 molar teeth easily fall out. The Rodentia always have two long 

 chisel-shaped incisors, covered only upon their anterior surface \vith 

 enamel, continually growing from behind, and implanted in very 

 long deep maxillary sockets ; behind the superior pair in Lepus 

 and Lagomys two lesser ones are found. The canines are here 

 wanting without any exception, and we therefore meet with a great 

 interval between the incisor and molar teeth. The herbivorous 



