hitherto referred to Coiiiliniis. 85 



to foini tlic piTspiit aiitcrd-iiiternal and in<!tlian internal cinps. 

 Ilowcvt-r tliis latter poirit maybe — ami even after liavin;^ 

 had the ndvaiita^^e of discussion with Mr. Kiiiid AiidtM'.sen 

 over Dr. Win<,'c'.s theories I still find it rlitiicult to l)('li(;ve — I 

 certainly thiidc the postero-internal cusp cannot be ox|)lained an 

 he supposes. lie would call it 5 c, on the ground that it repre- 

 sents an ofV-spliitini^ of the inner corner of the main central 

 posterior cusp " 5" of the tooth as found in typical J/mv, ami 

 is therefore of very recent origin. But if we consider the 

 distriljution of the forms which possess it, scattered as they 

 are about the Old World- — Micromijs in the Palrearctic region, 

 ^^ Mus^^ arborarius and rutilatis in Africa, Grateromys and 

 Lenomys in the Philippines and Celebes, Pogonoinys and 

 llyovtys in New Guinea, — it seems impossible to believe that 

 these forms have all in wid(dy separated localities indopen- 

 <lcntly developed exactly the same structure from a type, as 

 1 suppose, so advanced and certainly so dominant as Mas. 

 It would rather appear natural to su|)p03e that (even if a 

 later growth as compared with the very primitive Cricetinc 

 St ries of JMurida') it is an early develo[)ment within the true 

 l\Iuiin;e, occurring here and there in the group, and has then 

 been reduced in some forms and lost in others, among which 

 latter would be the dominant and highly developed genus 

 Mus. 



Dr. Winge has shown himself such a genius in dis- 

 entangling the complicated homologies of molar cu=!ps that 

 it is with much diffidence that I put forward this modilication 

 of his views about this postero-internal cusp, which, for 

 purposes of discussion and not to prejudge the question, 

 might be called the x cusp. I would only recall that nearly 

 all the forms in which it occurs have been either discovered 

 or had their tooth-structure described since he wrote his work 

 on the subject, and that he therefore had Micromys alone to 

 examine, and was without the opj)ortunity of considering the 

 very important argument from distribution and occurrence in 

 otherwise widely se[)arated forms which 1 have now ventured 

 to bring forward. 



That many of the small additional cusps found in such 

 nmlticuspid Muridac as Leygnda^ Cliiraroinys^ and others are 

 leceiit suj)plemenfary additions, as Dr. Winge supposes, I 

 would freely admit; but I do not think this to be the case 

 with the interesting .c cusp, which is in Australia the 

 diagnostic mark of Amuiomys and Conilunis. 



