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Show 40 MR. M. F. WOODWARD ON THE [Jan. 5, specimens measure about 12-5 centim. long and only show a few of the larger hairs. The method adopted for the examination of the teeth is, I believe, the only reliable one (if employed in connexion with serial sections) by which the true relations of developing teeth in a young animal can be made out. The skin was carefully removed from the sides of the jaw, and the whole jaw, with gum covering it intact, was placed in absolute alcohol and thoroughly dehydrated ; it was then clarified in clove-oil, and either examined in that medium or mounted in Canada balsam. By this treatment the teeth are seen through the bones in their natural position in relation to one another and to the surrounding parts, whereas by dissection they are apt to be displaced. Further, some teeth when undergoing suppression are so minute as to be practically invisible to the unaided eye, so that under manipulation by ordinary dissection they would be entirely overlooked. On the other hand, if exclusively examined in serial sections it is doubtful if their exact position and relationships could be determined with absolute accuracy. This is notably the case with the anterior milk-incisors of the Rabbit (the upper of which measures only *13 millim. in length) described by Huxley (16) (figs. 4 & 5, di1), and which were discovered by this method, Huxley being, I believe, the first to apply it to the examination of tooth relationships. It is worthy of note that the method does not prevent one afterwards sectionizing the jaw ; in fact, it is best to examine the jaw first in this way in order to see exactly what teeth are present and what are their positions, as it then becomes much easier to interpret the appearances presented by serial sections. A microscopic examination of the clarified jaw (Plate II. fig. 1) shows that there are present at this age b teeth in the upper jaw and 7 in the lower one. Those of the upper jaw may be divided into premaxillary and maxillary teeth, there being 3 of the former (fig. 1, di\ di2, di3) and 5 of the latter (fig. 1, c, i., n., in., iv.). The first premaxillary tooth is very large and roughly conical; it lies completely buried in the bone and, from a comparison of a series of skulls, it becomes evident that it is the milk predecessor of the large permanent incisor; it attains a considerable size and persists for a long period after birth. As the foetus was very young, the permanent incisor had not yet been differentiated. The two posterior premaxillary teeth (fig. 1, di2, di3) are very-small, and variable ; both were present in three out of six preparations, one only was present in two, and both were wanting in the other. In two cases they were very fully developed (fig. 2, di2, dp), although small, and the anterior one was in all cases the largest. In the specimen figured the measurement of the anterior one {di2) is '62 millim. long X '35 millim., that of the posterior one {di3) being -31 millim. long X '19 millim. The anterior one presents a small conical crown composed of a distinct layer of enamel and dentine (fig. 2, di2, c, and d), a well- |