45 6 



ZOOLOGY 



but have well-developed chisel-shaped incisors which continue 

 to grow as they are worn at the extremity. The chisel edge is 

 preserved by the fact that the enamel is chiefly in front, and 



FIG. 240. 



FIG. 240. The Fox Squirrel (Sciurus ludovicianus) . Photographed by Folsom. 



FIG. 241. 



FlG. 241. Porcupine (Erethizon). By Dr. J. W. Folsom. 



Questions on the figure. To what order of mammals does the porcupine 

 belong? What are its peculiarities of habit and structure? 



the exposed, softer dentine behind is worn away more rapidly 

 by being used. The brain is smooth. The mammae are ab- 

 dominal. The rodents have world-wide distribution, and are 



