"'■: ' V '- 1 ' -■ 



ROCKY MOUNTAIN PACK OR WOOD RAT 



(Neotoma cineria) 



the trade mark, or to so word the rule 

 that it will apply to all cases. Hares, 

 for instance, classed as rodents, are 

 inconsiderate enough in some of their 

 species to grow no less than six upper 

 teeth. Of these one pair is shed when 

 the animals arrive at years of discre- 

 tion, but another entirely useless pair 

 is retained hidden behind the two large 

 front serviceable ones. 



"Seen from the front the two super- 

 fluous teeth do not appear, and so the 

 rule seems to hold good. 



"There is, no such bother about the 

 other incisors ; no living rodent has more 

 than one pair of fronts lower teeth. 

 I am interested in teeth (the doctor is 

 a dentist) and among all the teeth that 

 I know anything about a muskrat's 

 teeth are the most interesting. One 

 curious fact about them is that they 

 keep on growing as long as the animal 

 which possesses them lives." 



"In that case," I said, "the oldest 

 muskrats ought to have the longest 

 teeth." 



"No," replied Doctor Lawrence, "the 

 rodents keep them worn down to their 

 proper length by constant use. They 



are the finest tools possible for the work 

 that a muskrat has to do — never wear 

 out, and are always sharp. You can 

 see what an edge the bevelled ends of 

 these incisors carry. The front surface 

 of these teeth is armored with the 

 hardest of animal substances, an en- 

 amel that rivals steel itself, while the 

 other parts, the sides and the back are 

 softer being composed of ordinary tooth 

 substance, dentine, or at best in some 

 instances, faced with a very thin ve- 

 neer of enamel. 



"Each tooth serves as a whetstone to 

 sharpen the one opposite to it, and it 

 follows of course, that the enamel wear- 

 ing off more slowly when the tooth en- 

 counters attrition, than the rest of the 

 parts subject to friction, preserves this 

 sharp bevelled edge. 



"Each incisor keeps the opposite one 

 from becoming overgrown ; it is curious 

 to see how the incisors continually tend 

 to become curved tusks." 



"What happens when one of the front 

 teeth is broken off?" I asked, "and fails 

 to check the growth of the tooth op- 

 posed to it?" 

 "In that case," replied the doctor, 



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