96 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



Fig. 46. — Dentition of Beaver {Castor 

 canadensis), m. 3, last molar, p. 4, 

 last premolar, i., scalpriform incisors ; 

 enamel face black, dentine in vertical 

 lines. 



the rootless, which are of simple form, like those of an armadillo, 

 and grow throughout life, never forming roots. The chisel-like, 



or scalpriform incisors of the ro- 

 dents do not cease to grow while 

 the animal lives ; they are kept 

 of constant length by continual 

 use, and the arrangement of 

 harder and softer tissue is such 

 that the sharp edge is main- 

 tained; through accident or 

 malformation it sometimes hap- 

 pens that the upper and lower 

 teeth fail to meet, then the con- 

 tinued growth causes them to form curved hoops in the mouth, 

 locking the jaws and bringing death by starvation to the un- 

 fortunate animal. 



The typical mammaUan tooth is composed of three kinds 

 of tissue, all differing in structure and hardness and called 

 respectively (1) dentine, (2) enamel, (3) cement. (1) The 

 dentine, or ivory, is the indispensable tissue of the tooth ; the 

 other kinds may be absent, but never the dentine. Chemically, 

 it is like bone, but the microscope shows that its structure is 

 quite different from that of true bone, being composed of an 

 immense number of fine tubules, which radiate from the 

 "pulp-cavity," or chamber which contains the blood-vessels 

 and nerves, these entering the tooth through the canals of the 

 roots. The tubules of the dentine lodge excessively fine 

 fibrillse of the nerve and that is why the cutting into a Uve 

 tooth is so painful an operation. (2) The enamel, which is 

 the hardest of all animal tissues, has a polished and shining 

 appearance and is arranged in a mosaic of microscopic prisms, 

 closely packed together, which in most mammals are solid, 

 but in the marsupials, with some exceptions, are tubular. 

 The enamel normally covers the entire crown of the tooth, 

 but does not extend upon the roots, where its superior hardness 



