STEUCTDRE OF THE HAIRS UF MTLODON LlSTAt. 401 



across. It is no uncommon thing to find a ragged flap of 

 cuticle overhanging the groove, as at h in fig. 8. These tacts 

 tend to show that the grooves are subcuticular ; Welcker, in 

 fact, goes so far as to state (17, p. 56) that the cuticle bridges 

 over the grooves except in certain places, and his fig. 21 

 lends support to this view. And yet there is no denying the 

 fact stated above, that the cuticle can be traced down the 

 side of the groove. The logical conclusion, therefore, to 

 which these facts point is that the grooves are morpho- 

 logically intra-cuticular, a view which is in complete accord 

 with the third suggestion offered in the case of the extra- 

 cortex of Bradypus — that the cells are those of thecuticular 

 layer, more numerous and less cuticnlarised than usual. 



The hairs of Choloepus are as a rule coarse, and with a 

 single curve extending over the greater part of the length, 

 while the basal fourth or so is wavy ; but in young specimens, 

 aiid in some apparently adult specimens from Costa Rica, the 

 hair is very delicate and soft, and sinuous from base to point. 

 The differences may be specific,^ or due to age, season, or 

 sex. However, in these forms the hairs are only about 42 /x 

 across, and have only two or three furrows instead of the 

 more usual nine, ten, or eleven. The algae, also, are quite 

 absent from many of the grooves. When such an empty 

 groove is examined in optical section (fig. 12) it exhibits the 

 outlines of obsolete extra-cortical cells, the edges of which 

 are conterminous with those serrations of the margin which 

 indicate the edges of the cuticular scales. In baby specimens 

 more than half of the hairs are slender, non-meduUate cylin- 

 ders, with very distinct scaly cuticle, and no grooves on the 

 surface. They are only slightly shorter than the two- or 

 three-grooved hairs just referred to, and constitute the 

 nearest approach to an under-fur found in Choloepus. 



' 'i'lie species didactylus and HolTnianni were supposed to differ in the 

 number of cervical vertebrae. Although this distinction has broken down, 

 Choloepus Holfmanni may still prove to be a good species. Until more 

 accurate knowledge is available concerning the geographical range and internal 

 anatomy of the so-called species of Choleepus the point must remain open. 



