34 MICROTIN.E 



ossified. In the pelvis the pubic symphysis is characteristically 

 shortened. 



Many of the interpretations of Microtine structure given in the 

 foregoing paragraphs have already been made by Winge.^ I 

 have, however, worked through and reflected upon the whole 

 subject for myself many times, and have arrived in my own way 

 at my own opinions and conclusions. These differ, in many 

 important respects,^ considerably from those of my illustrious 

 predecessor, to whom I am gratefully indebted for much kindness 

 and instruction, and it is therefore necessary for me to shoulder 

 responsibility for the whole. 



That the evolution of the Microtinse has proceeded along the 

 lines described above and for the general reasons given, becomes 

 evident not only when we review the known genera, but when we 

 follow the post-natal development of the individual. Comparison 

 of unworn tooth-germs with specimens in successive stages of wear 

 confirms the views expressed above as to the character of the 

 dentition of the ancestor and with regard to the manner in which 

 the ancestral molars have been modified (see the special sections 

 dealing with the dentition at pp. 99-124). Similarly the post- 

 natal growth of the skull in various genera recapitulates many of 

 the chief evolutionary processes which have resulted in the skull 

 forms characteristic of the adult stages of growth in the most 

 highly specialized genera. For example, new-born skulls of 

 Dicrostonyx, Evotomys rufocanus, Arvicola, and Ondatra (Plate II) 

 are all much alike and closely similar to the skull in adults of the 

 most primitive Muridse. They naturally show the beginnings of 

 the Microtine specialization ; but most of the essential characters 

 of the group become more and more evident in later stages of 

 growth. 



In two respects the Microtinse, judged by their most primitive 

 forms, stand lower in the scale than all other Muridae. Firstly, 

 their cheek-teeth retain more of the primitive longitudinal com- 

 plexity than do those of any other subfamily. Secondly, in the 

 lowest Microtinse, the Lemmi, the lower incisor is unusually short, 

 not extending backwards beyond m^, and is wholly lingual to the 

 molars in position. In all other respects, the Microtinse have 

 been carried by the two-fold specialization described above to a 

 level far above that attained by any of those members of other 

 subfamilies of Muridse that have specialized in somewhat similar 

 directions. 



^ WiNGE, " Om Graeske Pattedyr," Vidensk. Medd. Naturhist. Foren. 

 Kjobenhavn, 1881, pp. 36-50. " Jordfundne og nulevende Gnavere 

 (Rodentia) fra Lagoa Santa," E. Mus. Lundii, 3, pp. 123-126, 1887. 

 " Gronlands Pattedyr," Meddelelser om Gronland, 21, 1902, pp. 358-360, 

 382-386. " Danmarks Pattedyr," 1908, pp. 50-55, 68-79. " Pattedyr 

 Slaegter," 2, 1924, pp. 37-45, 137-143. 



^ As long ago as 1914 Winge and I were comparing our views on this 

 subject, and he told me that I had got everything upside down. No 

 doubt others will be of the same opinion to-day ! 



