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Show 42 MR. M. F. WOODWARD ON THE [Jan. 5, incisors it is highly improbable that they could persist after birth, whence we may reasonably look upon them as destined to be in all probability either absorbed or shed in utero. Giebel and Brandt, as already mentioned, have described two Incisors on either side of the upper jaw of young animals, and it seems, therefore, probable that the 2nd incisor may persist occasionally, especially as in two of m y preparations, where only one of these teeth was present, the second incisor had undergone an increase in size, although it had not yet developed a fang ; in fact, it was much more in the condition of the other milk-teeth, being more normally developed. It will be observed from this that there is nothing which will justify the unqualified assertion of Giebel and others before alluded to, that two upper incisors normally exist in the permanent dentition; for these small teeth never, I believe, persist till the permanent teeth appear; although they are only represented in one dentition, I incline to the belief that they should be regarded as milk-teeth. In the lower jaw (Plate II. fig. 1), in addition to the 2 typical incisors and 4 premolars, we find on either side a small well-developed tooth (fig. l,c) situated between the incisors and the first premolar. It lies close to the surface of the gum and is intermediate in size between the two vestigial upper incisors, measuring *4 millim. long x '3 millim. wide, and is correspondingly well developed (fig. 2, c). It possesses well-marked enamel and dentine layers to the crown and a small simple fang. Like the small teeth in the upper jaw, its parts are all fully developed ; but it is so small that, as in the case of the 3rd upper incisor, it has never before been observed. It certainly never persists after birth, even if it ever cuts the gum at all. When the jaws are closed, this tooth is situated just between the upper canine and the third upper incisor, a position which suggests that it is the lower canine ; but it is so close to the lower incisor that it might very well be the missing tooth of that series. When, however, we note the order of suppression going on hi the upper jaw, we find that while the two posterior incisors rarely persist, the canine is occasionally present even in the second dentition ; this suggests that the latter is not so fully reduced as the incisor, wherefore we might, by aualogy, fairly expect to see the canine more pronounced in the lower jaw. Further, recent observers l find in the Rhinoceros, one of the immediate allies of Hyrax, where onlv one of the anterior series of mandibular non-cheek teeth remains, that that is in all probability the canine and not an incisor. From argument by analogy, I am therefore inclined to regard this small disappearing tooth as the lower canine, the 3rd and posterior incisor having apparently completely disappeared. From the foregoing it may safely be concluded not only that the canines have, in Hyrax, ceased to have any functional importance, but that the incisors are being reduced in number by the suppression of the posterior ones. In the permanent dentition the first premolars, both above and 1 Lydekker, in Flower & Lydekker (9). |