FOSSIL AND EECEXT LAGOMOKPHA. 447 



they may, in a sense, be termed primitive ; but, as a matter of coui'se, bracbyodont 

 and semibypselodont teeth, before they are perfectly developed, have the cavities at 

 their bases open as well as bypselodont teeth ; and when they are in this condition, 

 their brachyodonty is not yet " nn arret de developpement." Ontog-enetically and 

 " logiqiiement," every bypselodont tooth passes through a bracbyodont condition, the 

 shaft only gradually increasing in length. Phylogeuetically, brachyodonty is also more 

 primitive than hypselodonty, as is known to all scientific morphologists wlio have a 

 knowledge of paleontology. 



On PI. 39. tigs. 19 and 20, I liave delineated side by side in the anterior view a 

 posterior upper right premolar, p^, of Titauomys Fontaunesl — the same specimen 

 of wliich the upper view is figured on PI. 36. fig. 8 — and an upper right molar of 

 a young Pteromys, in which the roots are not yet closed. Fig. li< represents the 

 anterior view of a right upper molar of Tit. viseaovieusis, figured i]i upper view on 

 PI. 36. fig. 18. Now, if we are entitled to call roots, even though they be 

 imperfectly developed, the three prolongations of the crown in Pteromys (fig. 20), 

 I think we are justified in applying the same term to the evidently homologous 

 parts in the figured teeth of Tltanomys (cf. figs. 14 and 19, and figs. 1, 1, 5, and 13), 

 and in repeating what I have said formerly *, that the inner root of Tltanomys, which 

 ultimately will remain open, increases in size and receives a coating of enamel. 



Even perfectly adult bracbyodont teeth preserve at their extremity a minute 

 opening for the passage of nerves and vessels, so that it may be left to individual 

 judgment at which phase in the ontogeny or pbylogeny of a tooth we may begin 

 to use the term " root." Having no desire to juggle with words I would, be 

 quite ready to desist using this term for the part of the tooth of Tltanomys which 

 is the hoinologue of the inner root of Pteromys; but thereby nothing would be 

 altered. The question at issue is, whether or not a coating of enamel lias extended 

 to that part; and that this has been the case is shown [)lainly enough by the figtu'es. 



It is interesting to compare the tooth of Tit. vlsenooleiisis (fig. 14) with those of 

 Tit. Fontannesl (figs. 1, 13, 19). The small outer roots are perfectly closed in the 

 former and more detached from the shaft than in the latter. The tooth of the 

 former, as shown by the upper view (PI. 36. fig. 18), is from an old individual; but in 

 none of the numerous upper premolars or molars of Tit. Fontannesl have I met with 

 closed outer roots. The coating of the enamel does not extend so far downward on 

 the inner side in Tit. vlsenovlensls as in Tit. Fontannesl. 



A further diff'erence between the Lower and the Middle Miocene species is also 

 characteristic. In the former (PI. 39. fig. 14) the external part of the crown extends 

 more outward than in Tit. Fontannesl, beyond the small roots ; this character has 

 been already noticed and explained in the description of the triturating surface, as 

 due to the atrophy of the outer region being less advanced in Tit. olsenoclensls than 

 in the more recent species. 



To proceed now to a closer examination of the small outer roots of the upper molars 

 and premolars of Tltanomys. In a passage, quoted above, p. 4.10, from H. v. 



* Proc. Zool. Soc. London, 1S93, p. 206. 



