A/n'ctin small MatnmaU. Ill 



never found in place in a skull, owing to the falling out of 

 tlio anterior one^ before the posterior ones conic up, so that 

 not a ainj^le skull in the considerahle Miiseuni collection has 

 six teeth, the best specimen (No. i)0. G. 8. 30) hiiving five anJ 

 an empty alveolus behind in which the sixth was to be 

 developed. But it is not clear that this sixth tooth (m^) ia 

 alwiiys developed, and I tlieret'ore arguo that it is the first 

 tooth of the set to be sup[)ressed, all the more that its situation 

 on the root of the great ijicisors reduces its freedom of 

 development and use. Next to this it mi;j;ht l)e argued, for 

 the same reason, that vr had disappeared ; but I prefer to con- 

 sider, although without real proof, that p' is tlie next to be 

 suppressed, as in quite young specimens, with unworn teeth, 

 it is smaller than the one next it, while in equally young 

 Qeorychiis the two anterior teeth are praciically equal in size. 

 From this, therefore, cutting off a tooth at each end ol" the 

 series, it would follow that in the 4-toothed members of the 

 family, Georychus and Bathyergus, the formula is P. 3 . 4, 

 M. 1 . 2. 



Then in Georychus, of its four teeth, it is always the last 

 which is the smallest, and this is situated quite on the large 

 incisor-root, so that there seems no doubt that it is this tooth, 

 r/»*, which is the next to disappear, leaving the three-tooth 

 formula of Ileterocephalus as P. 3 . 4, M. 1. 



And then again, for the same reasons and even more 

 indubitably, it is the last of these, vi 1, which disappears to 

 leave the extremely specialized reduced formula of Fornarina 

 ihilUpsi* : P. 3.4, M. 0. 



It is therefore interesting to notice that while Fornarina 

 shares with llydrotnys the extreme reduction to two cheek- 

 teeth, these are not homologous, those of Fornarina being 

 P. 3 . 4, and of Ilydromys M. 1. 2. 



Put into tabular form the tooth-formulae of the group would 

 be as follows : — 



JItliophohius : 1. , , P. 2~':374> ^^- i 

 Bathyergus'.\ j i p 3^4 »» i 

 Georyclius: f '' ^' '•«•■" ^'' i 



Ueterocej>halus : 1 ^, P. g-^, M. -j 

 Fornarina : I.^, P. ^^ 



• One more recently ncfivi tl specimen of phil/ipsi Ims ft Piiinll tliird 

 tooth above, i. o. r/i', a' fuel whicli, iu conjunction with the nimectnnt 

 characters of JI. dunni, makes the validity of the genua Foniarinn very 



llnilbtful. 



