FOSSIL AND EECENT LAGOMOEPHA. 445 



crown's surface. In the figures which I give of the teeth of both forms *, no essential 

 difference is to be seen in tlie pattern. The disagreement in the description of the two 

 writers finds its explanation in the somewhat loose way of describing the triturating 

 surface, i. e. hj the failure to distinguish between a dentine surface bordered by two enamel 

 ridges which alternates with an enamel fold filled with cement, so that only the two 

 enamel borders of the fold appear on tlie surface. As an outcome of this alternation we 

 find, when proceeding from the inner side of the tooth to its outer side, the following 

 succession in the middle line of the tooth : enamel ridge ; dentine ; enamel ridge ; cement ; 

 enamel ridge ; dentine ; enamel ridge ; cement ; enamel tubercle of the outer side. 



Apparently the two writers do not always apply the term " chevron " to the same 

 thing. Filhol, speaking of the " deuxieme element " of the tooth, by whieli he means 

 the part of the crown backward from the anterior lobe, his " premier element," says : 

 " Chez les Titanomys, on peut le considerer comme constitue par un chevron a sommet 

 interne, dont les deux extremites circonscrivent une pointe externe. Ce premier chevron 

 est borne en dedans par un deuxieme chevron dont le sommet correspond au bord interne 

 de la dent. Ce mode de structure est surtout bien marque sur la troisieme molaire"!. 

 This description, which is quite correct as far as it goes, applies equally well to the 

 species of the Lower and to that of the Middle Miocene, as may be seen by a com- 

 parison of the figures (Pis. 36, 37, 39) ; by consulting the figures it may be further seen 

 that what the author calls chevrons are the spaces of dentine bordered by enamel ridges, 

 which spaces mark the position of enamel cusps before wear set iu. 



Deperet, in describing the same " troisieme molaire," I. e. the posterior of the three 

 premolars, of Laxjodiis Fontcmnesi, says :■ — " Cette couroune se compose de deux prismes 

 d'email etroitement accoles, un pen niieux distincts en dehors que du cote interne, qui est 

 de forme arrondie. Le prisme anterieur [Pilhol's premier element] est compose d'un 

 seul pli d'email transverse ; le prisme posterieur au contraire, a surface triturante coupee 

 obliquement en arriere, presente deux plis d'email en chevron a pointe interne, ce qui 

 dessine sur la couronne trois petits croissants concentriques, si Ton compte la lamelle 

 d'email qui limite le bord interne de la couronne "$. 



It is certainly not accurate to describe the single cylinder of which these upper teeth 

 consist as composed of two enamel prisms " etroitement accoles." Apart from this, 

 Deperet's descrij)tion, like Filhol's, applies to both Lagodus Fontcmnesi and Titanomys 

 visenomensis. By " deux plis d'email en chevron a pointe interne," the author evidently 

 has in view, firstly, the larger, internal, of the two enamel folds ; secondly, the crescent- 

 shaped cusp (6) external to it, which by the effect of wear presents a dentinal surface 

 bordered l)y an outer and an inner enamel ridge. By counting, moreover, the enamel 

 border of the internal side of the crown, Deperet arrives at the number of three " petits 

 croissants concentriques," which on the following page are called " trois plis en chevron 

 concentriques." Lilhol leaves oiit of account the enamel fold by which his two chevrons 

 are separated. 



* PI. 36. figs. IS, 19; PI. 37. fig. Tl; PL 39. fig. 16 (Tkanoiinjs vhemvlensis). PL 36. figs. 6-8. 12-1.5 

 .{T. Fontannesi). t Op. cit. p. 30. t Op. cil. p. 127. 



