1897.] MALAGASY GENTJS BRACHTTTROMTS. 711 



Bh. siualensis, speaking of specimens in the Indian Museum, 

 Calcutta: — " The relatively wider molars and the larger size of 

 the incisors of the existing forms as compared with those of the 

 fossil seem to indicate that the specialization has tended to the 

 production of these characters." ^ In a former paper ^ it had 

 been stated that " the fossil molars are in one specimen slightly 

 smaller, and in the other slightly larger than those of the recent 

 species" (i. e. Hh sumatrensis). 



In the mandibulse preserved in the British Museum, the molars 

 are wider and longer than those of the largest living species 

 (Rh. sumatrensis), although the mandibula is shorter and lower. 

 The elements constituting each of the molars in the recent forms 

 appear to be more crowded together antero-posteriorly than in the 

 fossil. In this last the molars are more complicated and remain 

 so even in a worn condition, cusp 1 not becoming fused with 4, an 

 occurrence which we meet with only in very young stages of some 

 of the species of living Rhizomys. Besides, as has been already 

 pointed out by Lydekker, in the fossil molars the external fold 

 has a greater depth than in recent species. To sum up. The 

 molars of Uh. sivalensis preserve the main characters of the molars 

 of young specimens in later age, whilst in all the recent species 

 the molars are very soon simplified, in some a little earlier, in 

 others a little later. 



Comparison of the Molars of the Hesperomyinse witJi those of 

 Brachyuromys and Nesomys. 



Amongst American Muridse the simplification of the pattern 

 of molars has been attained in two ways : (1) by a more or less 

 complete suppression of 1, 2, 3, accompanied by a predominance of 

 the four cusps 4, 5, 6, 7, notably the two former; so that the 

 crown of m. 2 remains essentially composed of four cusps ; (2) by 

 a more or less complete suppression of 2 and 3, whilst 4, 5, 6, 7 are 

 equally somewhat reduced in dimensions, whereas 1 is rather 

 increased ; atanyrate it has obtained quite or almost the dimensions 

 of 4 and 5. Of both molars Hensel and Winge have figured 

 examples. To the latter belong such genera as Sigmodon {his- 

 pidus), Holochilus (vidpinus), and the JVeotoniince. Whilst in the 

 former there exists a notable difference in form and size between 

 m. 1 and m. 2, as in the Hesperomyince with more complicated 

 molars ^, in the latter m. 1 tends to become more similar to m. 2. 



Of the first type, viz. brachyodont, bunodont molars, with 

 essentially only four tubercles, I have found no parallel amongst 

 living Malagasy Rodents ; but I discovered in the lower deposits 

 of the " Children's Cave," near Sirabe (Central Madagascar), some 

 jaws belonging to this type ; these wiU be dealt with on a future 

 occasion. To the second type belongs, amongst Malagasy 

 Rodents, Brachyuromys ; and from other parts of the Old World, 



1 Pal. Ind. I. c. p. 108. ^ Rec. Geol. Siirr Ind. xi. 1878, p. 101. 



^ Of fourse, as was shown so long ago ns 1873 by Hensel, there are all 

 possible transitions to be found between the simple and the more complicated 

 molars. 



