FOSSIL A^"D EECEXT LAGO^rOKPHA. 473 



(2) From behind foncnrd in flic dental series. — Tlie true molars are the first to lie 

 transformed, and successively one after the other of the premolars, the anterior pre- 

 molar (p. 3) being the most conservative. 



(3) From young to old. — Tlie cheek-teeth of the genera under consideration exhiltit, in 

 the first developed jiarts of their shaft, more or less evident traces of the ancestral 

 pattern; mostl)/ so the deciduous teeth, which are cast olT Avlien the primitive pattern 

 has almost vanished, and without sliowing a beginning of transformation ; least so 

 the true molars, \Ahich in the first stages observable of the calcified tooth, and before 

 trituration has set in, show the primitive pattern already reduced and the secoudaiy in 

 process of evolution. 



Lower Molars of Lagomorpiious Rodextia. 



To state it in a general way, the lower molars of the Lagomorpha present the 

 same characters as their upper antagonists : viz. anteriorly in the series we meet with 

 complication, posteriorly with a simple transverse j)attern. On closer examination, 

 however, it may be seen that in the mandibular teeth the process wliich we have followed 

 through its various stages ia the upper set is accelerated. Although it must be taken 

 into account that we have one premolar less below than above, none the less — leaving for 

 the present out of consideration the reduction which takes place at the posterior end 

 (m. 3) — there is in the adult mandible only one tooth, the anterior, which diffei's materially 

 from the others, by being more complicated. In Titanomys, the oldest member of the 

 groitp, this tooth (p. 2) as generally described and figured, presents a more simple 

 structure tlian in later genera, and even than do the other teeth of Titanomys, by being 

 composed of only one column, divided into two lolies by an inner and an oiiter enamel- 

 inflection ; whereas in the teeth situated posteriorly there are two columns, the division 

 between them being complete ; tliey are held together by cement. 



"We meet here with a phenomenon which is pretty general among Eodents, whether 

 the number of their cheek-teeth be three, four, five, or six. To state it more fully : — 



1. The mandibular cheek-teeth precede those of the maxilla in the reduction of their 



number; mc have instances of .', of '' and of .. cheek-teeth, but never of "!, or ;, or j. 



2. Very frequently the anterior tooth in the lower series, whether it be p. 2, or p. 1, 

 or m. 1, is more complicated than those behind; which circumstance suggests that the 

 complication has some connection with the anterior position of the tooth in question. 



3. When the anterior lower tooth is nearly or actually equal in pattern to those behind, 

 this is generally so in older forms. Thus we find that in Winge's Anomaluridae — 

 including mostly Tertiary genera — provided with four lower teeth, the anterior one (p. 1) 

 is equal or subequal in size and pattern with the others, and sometimes even of smaller 

 size. Again, in Muridte, with three inferior cheek-teeth, the geologically older forms 

 have the anterior one (m. 1) equal or subequal in size with the two following, whereas 

 the complication of the first molar appears only in more recent forms. The same is 

 true with regard to the lagomorphous Rodentia, where the anterior tooth is p. 2, and 

 in the oldest known genera {Titanomys, Fala;olagiis) of a rather simple pattern, 



second SKRIES. — ZOOLOGY, VOL. VII. 66 



