THE SKELETON TN MAMMALIA. EXOSKELETON. 



apex fray out into long fibres which look like hair. At the 

 inner edge of each principal plate are subsidiary smaller plates. 

 The plates are formed of a number of fibres each developed 

 round a dermal papilla in the same way as are the fibres 

 forming the horns of Rhinoceros. Baleen and Rhinoceros 

 horn likewise agree in that the fibres are bound together by 

 less hardened epithelial cells, which readily wear away and 

 allow the harder fibres to fray out. The greatest develop- 

 ment of baleen occurs in the Northern Right whale, Balaena 

 mysticetus, in which the plates number three hundred and 

 eighty or more on each side, and reach a length of ten or 

 twelve feet near the middle of the series. 



DERMAL EXOSKELETON. 



Mammals show two principal kinds of exoskeletal struc- 

 tures which are entirely or partially dermal in origin, viz. the 

 bony scutes of armadillos, and teeth. 



The bony scutes of armadillos are quadrate or polygonal 

 in shape and are in general aggregated together, forming 

 several shields protecting various re'gions of the body. The 

 head is generally protected by a cephalic shield, the anterior 

 part of the body by a scapular, and the posterior by a pelvic 

 shield. The tail is also generally encased in bony rings, and 

 scutes are irregularly scattered over the surface of the limbs. 

 The mid-body region is protected by a varying number of 

 bands of scutes united by soft skin, so as to allow of move- 

 ment. Corresponding to each dermal scute is an epidermal 

 plate. In Chlamydophorus the scutes are mainly confined to 

 the posterior region where they form a strong vertically-placed 

 shield which coalesces with the pelvis. The anterior part of 

 the body is mainly covered by horny epidermal plates with 

 very little ossification beneath. In the gigantic extinct 

 Glyptodonts the body is covered with a solid carapace 

 formed by the union of an immense number of plates, and 

 there are no movable rings. The top of the head is defended 



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