480 DR. C. I. FOESTTH MAJOR ON 



Rabbit (fig. 8) ; the first lobe of the same is mainly composed of an outer and an inner 

 cusp, separated by a median hollow ; the second lobe is wrinkled as in p. 2 of tlie same 

 species. The minor cusps, though less distinct, are visible also in m. 1 and m. 2 of the 

 Eabbit, in m. 2 almost vanishing. I have noted tlieir presence in the true molars of 

 young specimens of other species as well {Lcpns europam, Lepns sp. from China, 

 Syloilagus brasiliensis) : t is generally present in unworn deciduoiis teeth, in premolars, 

 and in molars of several Leporidse. 



To sum up tlie above as regards the lower cheek-teeth, p. 2 — m. 2. An original 

 ari'angeinent into outer and inner cusps, sej^arated by a median longitudinal valley, is 

 traceable in the lower molars of Lagomorpha generally. It is more distinct in the 

 anterior cheek-teeth, and persists throughout life in p. 2 of most genera in both families ; 

 it is less distinct, though perfectly perceptible, in true molars, in which it very soon 

 disappears by wear, being replaced by the transverse arrangement. In p. 2 we have to 

 distinguish between an older complication and secondary additions ; the increase in the 

 plication alone is present in the posterior cheek-teeth, the anterior cusp not. On 

 comparing adult stages of p. 2 of Titanomijs witli the corresponding tooth of all other 

 Lagomorpha which, on the Avhole, are more recent forms, the latter appear to be more 

 complicated ; but in young stages p. 2 of Tiianomys Fontannesi presents also a com- 

 plicated appearance. This cannot be an incipient complication, for that joart of the 

 shaft of the tooth which is situated on the opposite end of the pulp-cavity is, as a matter 

 of coiu'se, always the oldest. llilgendorf has found the interruption of the enamel 

 border on tlie inner side also of lower molars of Lepiis *, a fact which points towards 

 a degeneration of this part of the tooth, and would seem to coll for a compensatory 

 increase on its outer side. However, I am ]:ot aware of a jjerceptible additional increase 

 on the outer side of lower molars of more recent forms, as compared with older ones f. 



Upj)er molars are more progressive than lo\^■e^ as concerning occasional additions. 

 An ingenious explanation of this general occurrence is given by Winge in the following 

 remark: — ' The explanation of the maxillary teeth making a larger increase than those 

 of the lower jaw is in all likelihood the following : they are placed in an unmovable 

 bone, where the conditions for nourishment are more favourable than in the compara- 

 tively slender and movable mandible " %. In our special case an increase of the lower 

 molars in the transverse direction can be the more dispensed with, since in the Leporidae 

 the movement of the jaws is chiefly lateral. This will not be denied by any one who has 

 ever examined the shape of their glenoid cavity or watched a Eabbit or Hare chewing. 

 Moreover, the dentine of both upper and lower cheek-teeth shows unmistakable signs of 

 this movement, in the piesence of ti-ansverse stria?, due to the action of the transverse 

 enamel crest of the opposite tooth. 



It remains to discuss in some detail the last molar, m. \'>, ahoiit which verv diver2:ent 

 views ha,ve l)een put forward. 



* Op. at. p. 23. 



t Neither am I aware of lacun.T on the internal enamel hoideriiic; of any Lngomvida^ ; but I must add that no 

 sections were made. 



I Vidcnsk. Meddelelser naturliist. Forciiing i Kjiihcuhavn f. Aar. ISSl?. p. ]7 (181^=3). 



