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Show 1892.] MILK-DENTIT!ON OF PROCAVIA CAPENSIS. 41 marked cervix, and a simple single fang, slightly constricted at its base. The smaller posterior one (fig. 2, di2) is much simpler and not so much calcified, and although it had no distinct fang yet the pulp-cavity was already partially constricted. In position the two teeth lie close to the surface of the gum, and though they overlap the premaxilla they are really external to it, alveoli being undeveloped l. The anterior one lies just in front of the premaxillo-maxillary suture (fig. 1, ms.), while the smaller posterior one lies just between the two bones, but under cover of the premaxilla and quite in front of the maxilla. In the two cases where only one of these teeth was developed, it was obviously the anterior one from its position ; it was larger and rounder than the one figured (figs. 1 & 2, di2), and was evidently younger and still undergoing development. The maxillary series of teeth are 5 in number : the first (fig. 1, c), a small uncalcified tooth-germ, which lies close to the anterior border of the maxilla, is obviously the canine ; while the four remaining ones (fig. 1, i., n., in., iv.) represent the deciduous premolars and have not yet developed their fangs. They are simple calcified cappings of the pulp, the only points of interest about them being their proximity to the premaxillo-maxillary suture and the manner in which the first one is displaced so as to partially overlie the second. Examining the teeth of the upper jaw as an entire series, we see at once that the most fully formed ones are the two small posterior premaxillary teeth {di2, di3). Unlike the remaining teeth, all of which lie deep down in the bone, these small ones are situated close to the surface of the gum, so that they must evidently cut the gum first if not absorbed. A tooth whose crown is once calcified does not, as a rule, increase in transverse diameter, because the dentine composing the greater part of it is deposited from within outwardly ; so that when dealing with a tooth possessing well-marked layers of dentine and enamel, one is fairly safe in saying that the crown will not enlarge much transversely. When the fang is formed and partially constricted at its base, one is justifiedin regarding that tooth as being fully formed, as the constriction of the pulp-cavity only takes place after the completion of the tooth. The small teeth (fig. 2, di2, di3), in possessing the above characters, may be safely regarded as having attained their full size and being ready to cut the gum. W e have now to determine the homologies of these teeth. As we have seen, they are situated in the premaxilla behind the large incisor and immediately in front of the canine. The only teeth they can represent are the 2nd and 3rd incisors ; so that the large incisor is thus proved from its position to be the first or anterior one. From the earlier development and small size of the 2nd and 3rd 1 Jaeger (JB. nat. Ver. Wurzb. 1860, xvi.) regards Cuvier's " trous incisifs " as the remainder of a deciduous 2nd incisor ; but as the 2nd incisor very rarely persists and is generally external to the premaxilla, it is improbable that this depression, which is fairly constant, can represent its alveolus. |
