426 University of California Publications in Zoology [Vol. 12 



The Dental Armature as a Cutting and Grinding Agency 



The work of the beaver, as exemplified in the cutting of materials 

 for lodges and dams, serves to illustrate the efficiency of the incisors 

 as cutting agents. The beaver's incisor is said to have been the 

 hardest substance except flint known and used as a cutting tool by 

 certain tribes of North American Indians. That the grinding por- 

 tions of the beaver's dental armature are no less efficient to perform 

 the function required of them is evident when it is considered that 

 bark, a substance requiring very powerful mastication, is the beaver's 

 principal food. Counting from front to back there are about 40 

 transverse cutting blades on each maxillary tooth-row, making 80 

 cutting blades for the upper teeth. A similar number obtains for 

 the lower teeth. Only one side at a time can be opposed in the process 

 of mastication, so that 40 blades above are brought against 40 blades 

 below in the course of one chewing movement. If there is enough 

 lateral motion during this movement, however, all 80 of the blades 

 of the upper teeth may be ground against the 80 blades of the lower 

 teeth. In the former case 80 cuts, in the latter 160, would be given 

 to the mouthful of material. In case the beaver makes 100 chewing 

 movements a minute, the number of cuts for that period would be in 

 the former case 8000, in the latter 16,000. 



Unfortunately, the writer has never been privileged to ascertain 

 from watching the animal in life what the characteristic jaw move- 

 ments are, nor have references to the matter been found in the litera- 

 ture examined. On the basis of the arrangement of the series of 

 teeth with reference to one another it may be concluded, however, 

 that there is an antero-posterior movement of probably 15 to 20 

 millimeters magnitude. That there is lateral motion is equally cer- 

 tain, although it must be much less than the antero-posterior. Eight 

 millimeters appears to be about the maximum sidewise movement 

 possible. 



Parallelism in Castor and Erethizon 

 The general resemblance of the enamel pattern of the cheek-teeth 

 obtaining between the genera Castor and Erethizon has been remarked 

 by former workers. Another character, of interest in this connec- 

 tion, is the condition of the palato-maxillary region, which is rounded 

 instead of plane in both genera. Authorities on classification agree 

 in referring the beavers to the sciuromorph section of the Rodentia, 



