46 MR. M. F. \VOODWARD ON THE [Jail. 5, 



IV. TJie Mil/i-Dentilion of the Rabbit (Lepus cuniculus). 



Although the Rahbit is so universally studied in our own labora- 

 tories and its auatoiriy is described in detail in so many of our 

 practical hand-books, not one of tlie latter rightly describes its milk- 

 dentition, in spite of the fact that all the details concerning it have 

 been long ago recorded. 



If one examines the jaws of a Eabbit from 2-3 weeks old (Plate II. 

 fig. 3), one finds on each side of the upper jaw 6 incisors arranged in 

 two linear series, 3 on each side of the middle line ; the anterior tooth 

 of each set is known to be the great permanent front incisor, while the 

 others have been variously interpreted. 



F. Cuvier (3), in the first place, described them as representing 

 the 2nd and 3rd upper incisors ; his statement has been copied by 

 several authors (9, 15, 29), who have thus ascribed to the Rabbit at 

 birth 3 incisors, stating that the outer one is soon lost. 



This determination of Cuvier's was refuted by Owen (23) as long 

 ago as 1868 and later by Krause (17). These two observers show 

 that the middle tooth of each series (fig. 3, cli^) is in reality the 

 2nd milk-incisor ; it is a functional tooth for the first three weeks 

 of the animal's life, after which time it is shed, being pushed out 

 by its successor (fig. 3, pi'). The deciduous tooth in the specimen 

 fio-ured is small and wedge-shaped, its crown being much worn, while 

 its successor presents a conical unworn extremity, having only just 

 cut the gum. This latter tooth is the one described by Cuvier as 

 being the 3rd incisor and by others as being early lost (9, 15, 29). 



The probable reason that the 2nd milk-incisor and its successor 

 are present for some time side hy side, after the latter has cut the 

 gum, may be implied in the fact that the deciduous tooth is as it 

 were wedged in between the great anterior incisor and its own 

 successor and is rather worn away by attrition than shed. 



The study of the development of these teeth shows at once that 

 these two (fig. 3, dr, pi^) are formed from a common enam.el organ, 

 and that they possess the relations of a typical tooth of the 1st to 

 its successor in the 2nd dentition. The fact that the permanent 

 tooth cuts the gum posteriorly to the milk-tooth, instead of develop- 

 ing underneath the latter and gradually pushing it out, goes for 

 nothing, when we consider that the permanent tooth is typically 

 developed on the inner side of its milk predecessor and not below it. 



We see from the above that there are only 2 incisors on each side 

 of the adult upper jaw, and no examination of even the youngest 

 foetus in which the teeth are appearing shows us any trace of a 

 3rd one. 



The deciduous premolars of the upper series are 3 in number, 

 and, as may be seen, they persist until the animal is between 3 and 4 

 weeks old (not, as stated by Marshall and Hurst (22), being shed 

 before birth). These teeth have been long known and are figured by 

 Owen (23) ; the principal point of interest about them is tiieir 

 possession of true fangs and their replacement by more specialized 

 teeth which grow from persistent pulps. 



