512 THE AMERICAN NATURALIST [Vol. LIV 



under examination, since the form of the scale (more 

 especially) undergoes alteration from the base to the tip 

 of the hair shaft. As a rule the scales at the base of the 

 hair are of greater longitudinal than transverse diam- 

 eter, while the converse is true of the scales at the tip of 

 the shaft. Figs. 172, 173 and 174 illustrate the nature of 

 the change in form which is normally met with in the 

 hairs of mammals as it occurs in the fur hair of the pla- 

 typus (Ornithorhynchus anatinus). This modification 

 in the form of the scales is believed to be due to the in- 

 creasing amount of wear to which the hair shaft is sub- 

 jected the farther away it is pushed from the follicle. 

 That external friction is the cause of scale alteration in 

 form is likewise suggested by the fact that the stiffest 

 hairs possess, usually, scales of a much flattened type 

 (cf. Figs. 57 to 67, inc.), while the finer hairs show the 

 delicate, free ectal edges of the scales unchanged for at 

 least the proximal three fourths of the length of the 

 shaft. This is especially well illustrated in the hair of 

 the bats, notably in such species as the mastiff bat (Mo- 

 lossus sinaloa) (Fig. 105) ; the wrinkled-lipped bat (Nyc- 

 tinomus bocagei) (Fig. 106) ; and the intermediate bat 

 (Mormops intermedia) (Fig. 107). 



The fur hairs shown in the plates 6 were chosen with 

 the view of bringing out most clearly the nature of the 

 forms of the simple varieties of scales and medullas, and 

 of their various common modifications, as they exist one 

 third of the distance from the mouth of the follicle to the 

 top of the hair shaft. For convenience, therefore, the 

 scales and medulla in this portion of the hair shaft have 

 been termed mature scales and mature medulla. The 

 scales at the distal extremity of the hair shaft, whose 

 modification in form is considered to be the result of 

 attrition, are called the attritional scales, and the pinched 

 out medulla of the same region, the fragmentary medulla. 



Inasmuch as the hair shafts represented in the plates 



"The fur hair shown in the plates were taken, where possible, from 



