18 POCKET GOPHERS OF THE UNITED STATES. 



incisor) and 4 smaller grinding- teeth behind (the premolar and molars). 

 All of the teeth are simple, rootless, tubular prisms, closed at the top 

 and open at the base. In life the lower part is filled with a soft pulp- 

 like substance, supplied with blood vessels which replenish the tooth 

 from below, enabling it to grow as long as the animal lives. The 

 hardening of the pulp within the tooth forms the dentine and osteo- 

 dentine; the enamel and cement are deposited on the outside. In the 

 adult the enamel is attached to the outside in the form of vertical 

 plates or bands like staves on a barrel. 



Each, incisor is provided with a single band of enamel, which covers 

 the front face of the tooth and forms the chisel- like cutting edge. The 

 premolar has 3 or 4 plates. The true molars have 2 each in the genus 

 Thomomys. In the genus Geomys the last upper molar always has 3 

 plates and the lower molars 1 each. The upj)er intermediary molars 



Pig. 4.— Outline of skull of Mexican Gopher, showing teeth in normal position 



have 2 each in Geomys, but only 1 in Baird's Gopher (Cratogeomys 

 castanops). The free ends of the enamel plates project slightly beyond 

 the rest of the tooth, forming effective cutting blades. 



All of the teeth are attached to the sockets by means of vertical 

 cushions which extend from root to gum. This method of attachment 

 not only relieves the tender pulp at the base of the tooth from pressure, 

 but gives to the cutting edge an elasticity that is highly effective. 



The teeth as a cutting machine. — The destructiveness of the Gophers 

 in cutting and slicing roots may be better understood from a study of 

 the machinery that does the work. The upper front tooth or incisor is 

 used chiefly to anchor the animal to the root operated on, while the 

 lower incisor does most of the work, playing rapidly back and forth 

 like a steam drill until a piece is cut off and passed into the mouth, 

 where it is planed or sliced into tine particles, ready to be taken into 

 the stomach for digestion. The cutting edges of the enamel plates of 

 the molar teeth axe t^e tools that reduce tUe food, In the, Gophers, of 



