EVOLUTION OF MOLABS 107 



median tubercle (y), which caps the isthmus connecting the 

 posterior loop with the first inner triangle. This young tooth 

 thus shows that the posterior loop, closed triangles, and anterior 

 loop of the worn m^ of adult Dicrostonyx are direct derivatives 

 from the bases of the coronal tubercles, a median and an inner 

 tubercle forming together the foundation of each inner triangle, 

 an outer tubercle that of each outer triangle. 



The unworn m^ of Arvicola is of great interest. As long ago 

 as 1872, Forsyth Major called attention to the occurrence of 

 " accessory prisms " in very young and but slightly worn ex- 

 amples of this tooth ; ^ and in 1877 he gave a detailed account and 

 excellent figures of two of his specimens. ^ Although various 

 persons, including Winge, who accepted the characters of the 

 young Wj as evidence that Arvicola had descended from a form 

 that possessed a more complex dentition,^ have commented upon 

 Forsyth Major's observations, no one hitherto seems to have 

 studied the quite unworn tooth. Such extremely young teeth 

 are rare in collections, but I have managed during the past twenty- 

 five years to bring together a considerable number, from widely 

 separated localities in Europe and Asia, representing various late 

 Pleistocene and recent species. My material includes two or 

 three uncut germs and suffices to show that the complications 

 noted by Forsyth Major are constantly present in the early 

 stages of growth in this genus. 



When entirely unworn the m-^^ of Arvicola (Fig. 586, c) consists 

 of a posterior transverse loop, five alternating substantially closed 

 triangles, and a small but exceedingly complex anterior loop. 

 The enamel is seen in optical section as a thin sheet bounding the 

 greater part of the periphery of the tooth ; whether it extends 



1 Forsyth Major, Atti della Soc. Ital. Sci. Nat., 15, p. 122, 1872, Tav. 

 2, fig. 12. 



' Forsyth Major, Atti Soc. Tosc. Sc. Nat., 3, p. 117, Tav. ix, figs. 25 

 and 26. He says that in these figures " sono rappresentati due primi 

 denti inferior! di giovane Arvicola amphihiws, da me raccoiti nella . . . 

 Buca delle Fate sopra Molina di Quosa, nei quali si vede un piccolo prisma 

 accessorio tanto dal lato interno che dal lato esterno nella parte anteriore 

 del dente, di modo che il numero totale di prismi in questo dente h di cinque 

 air esterno e di sei all' intemo, come nella maggior parte delle Arvicole 

 [= Microti], mentre che nell' A. amphibiiis, come anche nell' A. nivalis e 

 glareolus, questo dente ha quattro prismi estemi e cinque intemi. 



" II paio di prismi accessorii pero e separato dal resto del dente soltanto 

 da un solco poco profondo, che per il logoramento presto sparisce. La 

 medesima conformazione 1' ho trovato in giovani primi denti inferior! dell' 

 A. nivalis; pero mai nell' A. glareolus." 



* WiNOE, Danmarks Pattedyr, 1908, p. 73, " Tilsyneladende er forreste 

 nedre Kindtand hos Vandrotten oprindeligere formet, med f;erre Slyngiiinger 

 paa Tvggefladen end hos de andre; men i Virkeligheden er den det ikke; 

 dens simple Form er fremkommen ved en Sammensmeltning og Udviskning 

 af Slynger, der hos Ungen endnu ere tydelige," and on p. 75 he says that Wi 

 " har hos det vo.xne D3'r kun 7 Emailleslynger ; men hos Unger kan der 

 vaere tydelige Tegn paa en Deling af den forreste Slynge i fiere." It is 

 interesting to find Winge recognizing the validity of the principle maintained 

 80 ably and for so long by Forsyth Major. 



