1866.] LORD — MUSK-RATS AS BUILDERS. 47 



grinders have each two on either side ; the angles are alternate. 

 The npper cutting, or incisor teeth, are broader than the lower, 

 plane in front, but bevelled off at the outside edges, the lower 

 being more rounded away than are the upper. Like the teeth of 

 all the rodents, they are admirably constructed chisels, that by a 

 simple arrangement of hard and softer material, sharpen them- 

 selves, the cutting edges becoming keener in proportion to the 

 density of the material gnawed. The musk-rat's mouth is truly a 

 marvellous mill, worked by machinery that needs neither steam or 

 water-power to drive it. Its millstones — by the side of which 

 man's best contrivance is but a bungle — never wear smooth, nor 

 deteriorate in grinding capabilities, however hard the ' miller ' 

 works. To supply the mill are admirable nippers that never 

 blunt, and always remain the same length, wear and growth being 

 so admirably balanced. 



A very marked peculiarity in the skull of the musk-rat is the 

 curious shape of the temporal bone ; so compressed is it betwixt 

 the orbits as to narrow the skull into a mere isthmus, not at all 

 wider than the extreme end of the muzzle. Parietals very small ; 

 occipital foramen nearly circular. 



Fiber osoyoosensis Lord. Sp. char. — In total length 3J 

 inches shorter than Fiber zibethicus Cuy. ; in general size 

 much smaller. General hue of back jet-black ; but, the hair bein^ 

 of two kinds, if viewed from tail to head, it looks grey — the under 

 fur being fine, silky, and light grey in colour ; concealing this on 

 the upper surface are long coarse black hairs ; the belly and sides 

 somewhat lighter ; head broad and depressed ; neck indistinct ; 

 ear small, upper margin rounded ; eye small and black ; whiskers 

 long, and composed of about an equal number of white and black 

 hairs ; incisors nearly straight, on the external surface orange- 

 yellow. The thumb of the fore-foot is quite rudimentary ; the third 

 claw is considerably longer than the second and fourth. The 

 hind feet are singularly twisted, the inner edges being posterior to 

 the outer. This simple modification of position, gives the animal 

 immense power in swimming. The feet are then bent towards 

 each other ; in the backward stroke, the full expanse of the flat 

 soles pushes against the water, sending the swimmer forwards ; in 

 the forward stroke the feet are ' feathered,' like rowers feather an 

 oar, passing through the water edge on, offers the least possible 

 resistance. The claws on the hind feet are small, compressed, 

 and but slightly curved. The skin covering the under surfaces 



