554 BALDWIN SPENCER AND GEORGINA SWEET. 



times of the year in the case of the same individual tlie 

 numbers will vary. 



The large hairs are easily distinguishable from the spines, 

 and this feature is most marked in the case of Central Australian 

 specimens, owing to the fact that whereas the spines are always 

 circular in section, the large hairs are flattened (figs. 7, 8), 

 though there is not the distinct division of the hair into haft, 

 shield, and tip as in the case of Ornithorhynchus. 



The distinct wavy nature of the small hairs, especially of 

 those of the dorsal region, is a marked feature of Echidna, 



On the ventral surface of the body the small hairs vary in 

 diameter from *0357 mm. to "054 mm. ; the diameter of the 

 large hairs varies from -0612 mm, to '108 mm. On the dorsal 

 surface the small hairs average "126 mm., while the large spines 

 vary from 3*5 mm. to 4*5 mm., those of the large size being 

 scattered about amongst a larger number of the smaller ones, 

 though they are more thickly developed over two areas, one on 

 the dorso-lateral surface on each side of the body about half- 

 way back from the head. 



For further details of the adult hairs, reference may be made 

 to the descriptions of the figures. 



Muscles of the Hairs. 



In both Ornithorhynchus and Echidna a bundle of muscle 

 fibres is associated with each group of hairs. In the case of 

 Ornithorhynclius the fibres are, as usual, unstriated, but in the 

 case of Echidna the fibres are striated. The bundle, as 

 usual, passes from the superficial part of the corium obliquely 

 downwards, and is attached to the special modification of the 

 dermic layer which envelops the group of hair follicles; or, in 

 the case of the large hairs which develop into spines and are 

 not accompanied by small hairs, the muscle is attached near 

 the bottom of the follicle, there being no special development 

 of the outer root-sheath to form a swelling at this part. 



From their disposition these striated fibres are evidently 

 the equivalents of those forming the arrector pili in other 

 mammals, and the only other instance known to us of the 



