AN UNUSUAL EXAMPLE OF INCISOR GROWTH IN THE 

 WESTERN FOX SQUIRREL. 



DAYTON STONER. 



A short time ago a partly broken skull of the Western Fox 

 Squirrel, {Sciurus uiger rufiventer (Geoffroy), bearing a curi- 

 ously formed upper incisor was brought to me for examination 

 by j\Ir. W. F. Kubichek, now of the Children's Museum, Brook- 

 lyn, New York. The animal was evidently an adult and was 

 killed near Homestead, Iowa, November, 1917. The lower 

 mandible as well as the posterior portion oC the brain case is 

 missing. 



By referring to the accompanying fi^ires, the points made 

 in the following discussion may be followed more closely. 



As is well known, the incisor teeth of this squirrel and, indeed 

 of all rodents, grow from a persistent pulp ; growth continues 

 as rapidly as the tips of these teeth are worn away, and a sharp, 

 chisel-like edge is constantly maintained. Apparently this ani- 

 mal had, through some accident or other, lost the exposed tip 

 of the lower left incisor, thus leaving the upper incisor of that 

 side unapposed ; as a consequence, the latter tooth did not ex- 

 tend downward much farther than normally, but assumed the 

 outline of an incomplete circle, growing backward and upward 

 along the outer side of the superior maxillary. The apex of 

 the tooth at the time the animal was killed, had covered over 

 the anterior opening of the infraorbital foramen and a bony 

 deposit had begun to take place behind the tubercle and just 

 below and anterior to that foramen, as well as on the anterior 

 face of the orbital plate of the superior maxillary. Both these 

 osseous deposits were due, no doubt, to the continued and ever- 

 increasing irritation set up by the growing incisor. 



Conditions indicate that, after a time, the lower left incisor 

 grew out again, but instead of meeting the cutting edge of the 

 upper incisor of that side, it apposed the lower margin of the 

 now almost completed circle into w^hich the upper incisor had 

 grown. The lower margin of this recurved upper incisor thus 

 became worn into a thin edge at a point opposite the apex of. 

 the right upper incisor. 



As the lower left incisor continued to grow longer, not being 

 directly apposed by the cutting edge of a tooth in the upper 

 jaw, it was pushed to one side so that the outer face of the right 



