1 7 8 CASTOROI^OGIA . 



"The posterior extremity of the beaver presents a singular form- 

 ation. The body diminishes in size gradually from the hips, and 

 terminates in a flat scaly tail, which, measured from the sacrum, is 

 about i8" in length; the first 8" being covered with hair like the 

 rest of the body. The scaly portion commences abruptly with a 

 width of about four inches, and terminates with a rounded extremity. 

 The scaly portion is slightly convex above and below, thin at the 

 margin, and is covered with a black, tough, scaly epidermis. The 

 scales are somewhat irregular in form and size, the most usual form 

 being sub-hexagonal, about 32'" in length, and 12'" in width. They 

 are arranged transversely in respect to length, in the so-called quin- 

 cunx form, and they diminish in size toward the end of the tail ; 

 across the middle of the tail the number is 19 or 20 above, and 20 or 

 21 on the under surface. A few short, broken hairs pass out be- 

 tween the scales. 



' ' The tail is composed largely of a dense fatty tissue, upon this 

 lies the derm or skin, its outer surface being serrated, with the points 

 of the serratures toward the end of the tail. Over the serratures is 

 extended the tough, horny epiderm, which is inflected under the 

 serratures, so as to present the imbricate appearance." 



Some idea of the popular conception of the beaver's tail can be 

 gathered from early illustrations, and particular attention is called 

 to the accompanying figure, and also to the several reproductions 

 which appear in preceding chapters. 



The muscles of the entire order of mammals are studied and de- 

 scribed comparatively to the standard of the human frame. Modi- 

 fications in connection with certain peculiar functions occur in every 

 class, and these are the only parts necessary to consider in a mono- 

 graph such as the present. In the order Rodentia, and in the beaver 

 pre-eminently we have the development of the ' ' masseter muscle ' ' 

 in its highest degree as it is on this that the creature relies for its 

 power to cut and grind hard woody fibres. The extraordinary de- 

 velopment of the muscles to move the jaws gives the beaver's face a 

 full, rounded appearance, and not only are the tendons connected 



