552 BALDWIN SPENCER AND GEORGINA SWEET. 



In regard to tlie large hairs, Poulton (p. 158) says ''The 

 protective large hairs are evidently subject to much wear and 

 tear, and succeed each other very rapidly ; the new successional 

 hair, which is to be met with in nearly every section, emerging 

 from the same follicular mouth, in front of and therefore over- 

 lapping the base of the old one.'' In another part (p. 167) 

 Poulton says that " the appearance of two hairs in one follicle 

 is spoken of as an occasional appearance in other mammals. 

 In the large hairs of Ornithorhynchus it is the invariable rule." 

 A glance at the transverse sections figured by us (figs. 1, 2) 

 will show that no successional hair, such as is figured by 

 Poulton, is present ; nor, though we have examined both 

 transverse and longitudinal sections of the skin of three adult 

 animals, have we been able to find any successional hairs. The 

 same author, as quoted, says that '' the protective large hairs 

 are evidently subject to much wear and tear, and succeed each 

 other very rapidly." From the method of life of Ornitho- 

 rhynchus we think that the reverse is the case, and that they 

 are not subject to much wear and tear ; and further, that the 

 difference in regard to the successional hairs as described by 

 Poulton and ourselves is to be accounted for on the supposition 

 that there is not a constant but a periodic shedding and 

 replacement of hairs. Poulton's sections are on this supposi- 

 tion taken from an animal in which the shedding and replace- 

 ment was taking place, while ours are from an animal in which 

 replacement had already taken place and in which the hairs 

 were in the maximum state of development. 



With regard to the size of the hairs, that of the larger ones 

 (fig. 3) varies, of course, according to the part of the hair which 

 is cut through. In our sections the shaft — always round in 

 section — is cut through, the diameter varying from "0245 mm. 

 to '035 mm. ; the smaller hairs vary from "0071 mm. to 

 ■0105 mm. The latter measurement agrees closely with that 

 given by Meijere and Welcker, viz. "008 mm.; the size of the 

 large hairs given by Welcker is 048 mm., and by Meijere 

 •045 mm. 



In Echidna the hairs are arranged in groups as iu Ornitho- 



