343 



- 'T 



their blood sinuses, together with the increased amount of connective 

 tissue, are the causes for the elevation of this organ above the sur- 

 face of the cheek. 



Mole. — The protuberance in the cheek of the mole (Fig. 1) is 

 approximately three millimeters in diameter, and one-half millimeter 

 in height. It contains five tactile hairs (T), many 5, and numerous 

 C hairs. 



Rat. — While the common rat, Mus norvegicus, is somewhat 

 larger than the mole, nevertheless, the protuberance in the former 

 is not so well developed; for, instead of five strong vibrissae, there 

 are but three slender ones (Fig. 2, 

 jT). The other kinds of hairs, 

 however, are more numerous. 



White-footed Mouse. — The 

 white-footed mouse, Peromyscus 

 leucopus, approximately one- 

 fourth the length of a rat, has in 

 this organ one vibrissa (Fig. 3, T) 

 nearly equal in length to the 

 vibrissae of the rat. The other 

 two types of hairs are somewhat 

 less abundant. 



Bat. — On examining com- 

 mon cave bats, Myotis lucifugus, 

 the discovery of well developed 

 protuberances, each containing 

 two vibrissae (Fig. 4, T) was 

 made. The B hairs were fairly abundant, but the C hairs were al- 

 most entirely wanting. The protuberance was noticeably larger in 

 proportion to the size of the body than were those in any other ani- 

 mals except the mole. 



Opossum. — The protuberance in the opossum, Didelphys vir- 

 giniana, is of unusual interest. The enormously developed tactile 

 hairs, eight in number, measure seventy to eighty millimeters in length. 

 The hairs of type B occur sparsely or are wholly absent, while those 

 of type C are well represented. 



A comparison of these organs in the various animals examined 

 shows that in proportion to length of body the number of vibrissae 

 is greatest in the mole. As is well known, the anterior end of the 



Fig. 3. Fig. 4. 



Fig. 3. Protuberance of White-Footed Mouse. 

 Fig. 4. Protuberance of Bat. 



