FOSSIL AND EECENT LAGOMORPHA. 435 



As to the number of lower clieek-teeth, I find, as a rule, five in one of the species, 

 Tilanomys Fontannesi; but in two out of seventeen mandibular rami there are only 

 four teeth, there being no trace of an alveolus for the last small tooth, which probably 

 will be found constantly present in young specimens. 



In the other species, T. visenooiensis, the fifth lower molar is supposed to be oftener 

 missing than not. Pomel called Ampliilagus — regarded by him as a subgenus of 

 Lacjomiis — those specimens of T. viseuoviensis in which fi.ve mandibular cheek-teetii 

 were present ; those with only four teeth he placed in his genus Lagodus [Lagodits 

 picoides, Vomc\, = T'danomys visenomemis, H. v. Mey.). Eilhol has based a fusion theory 

 on the presence or absence of the small molar in question *. He assumes that at a certain 

 given moment there prevails a tendency to simplification in the Lagomyine dentition — 

 firstly by the fusion of the last (fifth) tooth with the penultimate, and secondly by the 

 tendency of the fused elements to disappear. 



This theory is at once disposed of by the fact that in the mandibles of Titanomijs 

 Fontannesi. before me both the fifth tooth and the posterior colonnette of the fourth — 

 which colonnette Filhol considers to be the fifth tooth fused to the fourth — are present 

 togetlier. I think that for T. visenovleusis the same explanation holds good as with 

 regard to T. Fontannesi, viz. tlie fifth tooth has sometimes been lost in the young animal 

 and its alveolu.s obliterated ; its frequent absence is simply explained by the fact that it 

 has dropped out in the fossils. 



Anyhow, the formula of Titanonii/s will have to be written as follows : — 



P.!!,M. A, or i'-^'P-'P-'="-'-™-- 



2' "-"• 2—3' "" p. 2, p. 1 ; m. 1, m. 2 (m. 3)' 



2. Prolagvs. — I have at my disposal the deciduous molars of two species of Prolagus 

 [P. fpningensis (Kon.) and P. sardus (Wagn.)]; there are three in the upper and two in 

 the lower jaw, as seen already by Fraas in the first-named species. In the skull of a 

 young P. sardus, where the deciduous teeth are in situ, the following may be seen : — The 

 anterior of the three deciduous teeth is not situated directly above the anterior premolar, 

 but slightly backward, closely appressed to the second deciduous, so that with its anterior 

 moiety it covers only the posterior part of the premolar ; besides it could not possiblv cover 

 the latter completely, being much smaller. It is needless to say that neither of the true 

 molars, both of which are already protruded in the skull under observation, supports a 

 milk-tooth ; as a matter of fact, the tooth called molar I. by Fraas, which in reality is the 

 posterior of the three premolars, is situated under the posterior of the three deciduous 

 molars, as is the middle premolar imder the middle deciduous. 



In the lower jaw of both species the two anterior of the four lower cheek-teeth replace 

 the two deciduous teeth. 



Therefore, since Prolagus has in the full-grown animal five cheek-teeth above and 

 four below, its tooth-formula will be : — 



P. ■', M. :-, or P-^-P-^-P-|='"-i-"^-^ . 



- 2 p. 2, p. i ; m. 1, m. •! 



* Aun. Se. Geol. x. p. 28 (1870). 



61* 



