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Show 1892.] MILK-DENTITION OF PROCAVIA CAPENSIS. 47 Completing the cheek-teeth above we find at the same age 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 paper printed in the ' Proceedings' of this Society, Prof. Huxley (16) in 1880 mentioned that he had discovered in the foetal Rabbit vestigial milk predecessors to the 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 fcetal Rabbits from the time when 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 this 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 {pi1) being the permanent anterior one and the smallest tooth (di2) being the milk 2nd incisor, the permanent one not being yet differentiated. The cheek-teeth are 4 in number and represent the 3 deciduous premolars (d.pm.) and the 1st molar (m1). In the lower jaw the large incisor (pij) is well developed, but there are only 3 cheek-teeth, viz. the two milk premolars (d.pm.) and the anterior molar (m^). In front of each of the large incisors (pi1, pij), above and below, is a small tooth (di1, dij, rather irregular in appearance, but with its crown composed of typical layers of enamel and dentine (fig. 5, di1, dix), 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 (di1) "13 millim. long x '09 wide ; the lower one (dit), 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 condition ; if carefully examined they are seen to present an irregular contour, and this I 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. |