18 



NORTH AMERICAN FAUNA. 



[no. 39. 



none of abdominal, and 1, 2, or 3 of pectoral. The usual number of 

 embryos ranges from 3 to 7. 



MOLT. * 



In the genus Tliomomys there seem to be normally two complete 

 changes of pelage during the year, from the winter to the summer coat 

 during spring or early summer, and from the summer to the winter 

 coat in fall. The time of these changes varies in different species and 

 often in the same species with different climatic conditions, and 

 apparently also with the varying physical condition of individuals, 

 as age and bodily vigor. In the northern and more elevated areas 

 the molt is more pronounced and its progress more rapid, owing to 

 greater extremes of climate; while in the equable climate of the 

 west-coast areas and in the Tropics of western Mexico very gradual 

 and slight change of pelage occurs. The progress of the molt is nor- 

 mally from the nose over the head and body to the base of the tail 

 along a crescentic line, a little faster on the back than on the sides. 

 On the belly the molt lags behind and is less regular. 



The spring molt is most conspicuous, as each succeeding wave of 

 hair is shorter than that displaced and leaves a dark line of plumbeous 

 basal color exposed on the old hair. The change is a creeping process. 

 New hair displaces old on the nose, apparently pushing out the old 

 and worn and taking its place along the line of advance backward 

 and downward. The line is normally a semicircle, but it often 

 becomes distorted by one part progressing more rapidly than another. 

 In some cases the change is complete as it progresses, all the old hair 

 being replaced, but in others the molt is only partial, a thinning 

 or filling-up process, but always progressing along the line in the 

 same way. In spring the first molt wave and often those up to the 

 fifth are but slight changes, a thinning, like the removal of one winter 

 shirt after another, but the final, perhaps fifth or sixth, change to the 

 thinnest summer clothing is a complete change as it progresses. The 

 number of molt waves is not easily determined from specimens, as 

 they often overtake and run into each other. The long winter coat 

 is retained on the ramp often until late summer, and the successive 

 molt waves strike against this until the short summer coat pushes 

 it off at the base of the tail just before, or sometimes, in high or north- 

 ern locahties, not until, the next winter coat has begun to make its 

 appearance on the nose. There are at least five waves in the spring 

 molt, as this number appears on many specimens taken in May or 

 June. Those taken later in summer usually show a less number of 

 coats but with more deeply marked division lines due to the combi- 

 nation of several waves into one or more. 



The fall molt is different. The winter coat is put on more rapidly 

 with the appearance of reaUy cold weather, and apparently the full 

 change is accomplished in two molt waves that begin at the nose and 



