474 DE- C. I. rOESTTH MAJOE ON 



The explanation whicli I suggest for these curious occurrences is as follows : — When an 

 anterior tooth drops out from the mandihle — generally through an apparent interference 

 of the incisor with its pulp — some compensation for its loss is necessary, as the corre- 

 sponding maxillary tooth is generally still in its place ; this compensation is brought 

 about by a complication on the anterior side of the tooth which has become the first in 

 the series by the loss of the origiually anterior one. Those genera which are nearer in 

 date to the epoch when the anterior tooth was lost will still present a less complicated 

 form of that which has succeeded to this position, while in the later genera the 

 foremost tooth will have acquired the complication. When p. 2 is dropped, p. 1 will 

 become the foremost tooth, and the same cycle will recommence, and so on. 



I next proceed to a closer examination of the lower cheek-teeth, starting from those 

 of Titanomys. A superficial comparison of the anterior tooth, jTli, of this genus, Avith 

 that of the other Lagomyida;, shows that in the former it is more simple than in 

 the latter, and presents an approximately tetragonal outline at its triturating svu-face ; 

 in FrolcKjus, Lagopsis, and Lagomys this is triangular (apex in front). Thus it 

 is that we find the tooth generally described ; but on closer, examination the matter is 

 somewhat more complex. I have figured five specimens of p. 2 of Titanomys Fontannesl, 

 from La Grive-Saint-Alban, in different stages of wear ; four are isolated teeth 

 (PL 37. figs. 1-4) ; the fifth is in its place in a left ramus, presenting the complete series 

 of two premolars and three molars (PL 37. fig. 7). Of T. visenoviensis I have one speci- 

 men, in a fragment of the right ramus, containing the two premolars (PL 37. fig. 25). 

 This species is from the AUier (Bravard Collection, Br. Mus. Geol. Dep. No. 31095). 

 The first stage in T. Fontannesi (fig. 1) represents a tooth which has not yet come into 

 wear. In the main it is composed of two lobes ; the anterior is subconical, the posterior 

 is much more extended transversely, and composed of a tapering outer and a thicker, 

 rounded inner cusp ; moreover, on the middle of its posterior surface appears a small 

 cusp {t) ; the anterior surface of this lobe is wrinkled. Even in this early stage the 

 separation of the two lobes is incomplete ; a ridge, running almost longitudinally back- 

 ward, from the middle of the posterior side of the anterior lobe towards the posterior, 

 shows that trituration Avould very soon have connected the two by a narrow isthmus of 

 dentine, thus separating from each other an outer and an inner enamel-inflection. This 

 we sec, in fact, brought about in the second stage (fig. 2). Towards the middle of the 

 anterior margin of the anterior lobe, a feeble cusp is visible in the first stage (1, fig. 1) ; 

 the same is more distinct in the second stage (1, fig. 2), where it is nearer to the inner 

 side. This cusp, to all appearance, is Wiuge's 1, Osborn's paraconid. Whether it 

 contains potentially some other element I must leave undecided ; as a mtitter of fact, 

 in the two teeth described, it does not occupy exactly the same position ; and in 

 T. visenoviensis (1, fig. 25) it is more approximated to the outer side. What is called 

 the paraconid is, however, somewhat inconstant in its position*. In p. 2 of T. vise- 



" Sue c.i/., the text-figuros in "W. D. Matthew, " A Revision of the Puerco Fauna," Bull. Am. Mus. Nat. Hist, 

 is. {l^'Jl). 



