190 Mr. M. A. C. Hiiiton on 



crown to its centre ; the less deeply re-entrant anterior valley 

 (fig\ 6b) lias its floor raised as a little rib. Each valley 

 contains a little cement. The hinder sides of the tnbercles 

 and the posterior valley are invested with a thin coat of 

 enamel, but wiiether tlie latter extends over the apices of the 

 tubercles or not is uncertain. The grooves of the out°r and 

 itnier surfaces — persi.-^tent features in adult incisors of Trogon- 

 therium — are seen to start from the summit. 



In discussing th"fe grooved npper incisors of Lagomorplia 

 Forsyth Major * has st ited that : — " Tlie incisors provided 

 with enamel-folds point back towards cuspidate incisors, for 

 the enamel-folds of lophodont and laminated teeth are obviously 

 the derivatives and lioniolognes of the 'valleys' separating 

 the cusps or tubercles." In this connection it is of very 

 great interest to find traces of the primitive complexity in 

 the unworn incisor of Trogontherium. It may be that in 

 some cases, where one or more grooves channel the anterior 

 faces of the incisors, their persistence is due to the fact that 

 such grooves are of use to the animal retaining them, as 

 TuUbergt suggests ; but, in my view, it is wrong to regard 

 such a groove as a new feature produced in any given case, 

 because it is there useful. It is a fact that frequently in 

 Lngomorpha and in other rodents more or less distinct traces 

 of a ])osterior incisor valley can be found in the form of a 

 shallow sulcus, wliich, devoid of enamel, can hardly be of any 

 functional importance. Further, in many voles vestiges of 

 vanished valleys can be found as narrow, shallow, well- 

 defined, and persistent grooves upon the sides of their 

 molars ; in these cases it is only by a study of unworn or 

 lirtle-worn teeth that the true significance of such, at first sight, 

 trivial features becomes a{)parent. 



One of the most interesting of Mr. Savin's recent acquisi- 

 tions from the West-llunton deposit is a foot-bone, which I 

 have dftermined as the right navicular of Trogoulheriuin — a 

 part hitherto unknown. In PI. VIII. Wg. 7 four views of 

 the fossil are given, with corresponding views (fig. 8) of 

 the navicular of Castor fiher. Generally the fossil closely 

 agrees with the recent bone in form and in the number and 

 arrangement of the facettes ; it differs principally in having 

 the articular surfaces much larger, the posterior spur, on the 

 other hand, much smaller — so that, as a result, its absolute 

 size is but little greater. Anteriorly it is wider, posteriorly 



* Forsytli Major, Trans. Liiiii. Soc, Zool. sor. 2, vol. vii. p. 470 (1899). 

 t TullbiTjr, ' Ueber dti.s Sy.stciu Jer Nairelhiero,' p. .'{Uo. 



