1892.] MILK-DENTITION OF PROCAVIA CAPKNSIS. 47 



Completing the cheek-teeth above we find at the sameage 2 molars 

 in use, and a 3rd developing one buried up in the maxilla. 



The lower teeth at this age call for no special comment ; there are 

 the characteristic inferior incisors (1 on each side), 2 deciduous 

 premolars, having the same characters as those of the upper set. 

 Behind these are 2 molars, the 3rd not having yet cut the gum. 



In a footnote to a paj)er printed in the ' Proceedings ' of this 

 Society, Prof. Huxley (16) in 1880 mentioned that lie had discovered 

 in the foetal Rabbit vestigial milk predecessors to tlie large upper 

 and lower incisors, thus making the full milk or 1st dentition to be 

 i. J, pm. -. This discovery has been generally overlooked by the 

 writers of works dealing with the Mammalia and the Lagomorpha. 



Having examined his preparations and subsequently worked out 

 this point in a number of foetal Rabbits from the time wlien the 

 teeth first appear until birth, I can entirely confirm Huxley's 

 statement. 



Fig. 4 is a drawing of the clarified jaws of a foetal Rabbit. 



At tliis period there are present 7 teeth in the upper and 5 in the 

 lower jaw. Of those in the upper jaw the first 3 are incisors, the 

 large tooth (pi^) being the permanent anterior one and the smallest 

 tooth (di-) being the milk 2nd incisor, the permanent one not being 

 yet differentiated. The cheek-teeth are 4 in number and represent 

 the 3 deciduous jiremolars {d.pm.) and the 1st molar (»»'). 



lu the lower jaw the large incisor (^J,) is well develoj)ed, but there 

 are only 3 cheek-teeth, viz. the two milk premolars {d.pm.) and 

 the anterior molar (Wj). In front of each of the large incisors 

 (/>i', pi^, above and below, is a small tooth {dl^, di^), rather irregular 

 in appearance, but with its crown composed of typical layers of enamel 

 and dentine (fig. .5, di\ di^), and possessing all the essential structures 

 of a tooth. In size these teeth are the smallest in the jaws and 

 measure as follows: — the upper one (df) "IS miilim. long X "09 

 wide ; the lower one (di^), which is much larger and varies somewhat, 

 being •19-*34 millim. long and •12-'16 wide. 



Examination of young jaws shows that these minute teeth are 

 among the first to develop, and when they appear they attain with 

 their enamel organs a relatively large size in proportion to the jaw. 

 Their growth is early arrested and they remain in a dwarfed con- 

 dition ; if carefully examined they are seen to present an irregular 

 contour, and this 1 regard as expressive of their partial absorption. 

 They are eventually forced out of the gum, about birth, by the 

 growing permanent teeth. 



These small teeth develop in common enamel organs with the 

 permanent cutting-incisors, of which they are, as Huxley pointed out, 

 the milk predecessors. As has been shown, they are never functional, 

 and, like the milk-premolar of the Guinea-pig (Cuvier), they are 

 shed in utero. 



The Rabbit, so far as we know, is the only Rodent possessing 

 milk predecessors to both incisors ; while it and the Common Hare 

 are the only Rodents known to possess deciduous incisors. 



