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Show 572 MR. M. r. W O O D W A K D O N [May 5, series, whereas in Centetes this tooth is developed nearly as soon as the undoubted milk-teeth and is shed about the same time as the members of that series. On investigating the development of JJ3 no indication whatever of a reduced successor is to be met with, the dental lamina being completely fused with the enamel-germ of this tooth, and consequently exhibits no lingual development. On the other hand, a slight outgrowth from the enamel-organ itself is visible on the labial side (Plate X X V . fig. 25, x), very similar to that figured by Kiikenthal (6) in the Walrus (Taf. iii. fig. 7, rvz.), and which he there regards as the remains of an earlier dentition. One might therefore be justified in regarding this structure in Centetes as the last trace of dUS, and the functional tooth though early lost as pi-3. I am, however, very doubtful as to the advisability of basing a conclusion upon such slight evidence, more especially as I have never observed an undoubted reduced labial tooth in such a position, vestiges of an earlier dentition being always, so far as I am aware, related directly to the dental lamina, i. e. to the neck of the enamel-organ of the replacing tooth and not to the modified body of that structure. Nevertheless, from the entire absence of any trace of a successor to this tooth and from the fact that the milk-dentition appears to be undergoing reduction in most Insectivores, and especially from the condition of the 3rd incisors in Gymnura and Erinaceus, I venture to suggest that this single i-_3 of Centetes belongs to the permanent dentition, but that it is very early developed and shed with the milk-teeth. It is interesting to note that in the closely allied genus ffemi-centetes a 3rd upper incisor is present in the adult dentition; but although we know a little of the tooth change in this form (3. p. 75), yet we do not for certain know if this tooth is preceded by a functional milk-incisor. The remaining incisors •'.'• V- ..together with the canines and the three premolars above and below are all present as functional teeth in both dentitions. A very marked gap is noticeable between the canines and the first functional premolars both above and below: this tends to confirm the generally accepted view that the missing premolar is the 1st of that series. Unfortunately the dental lamina has been completely aborted from this gap in both stages examined, so that no indication of a missing tooth could be found. The diastemata between these teeth are much more pronounced in the older stage and still more so in the adult; and from what I have seen in this and other long-nosed mammals (polyprotodont Marsupials), I am led to conclude that this elongation of the jaw is a secondary one, acquired since the reduction in the tooth series. This to m y mind accounts for the absence of all vestiges of the suppressed teeth, for, when recently suppressed, tooth-vestiges are generally found even in short-nosed forms, lhe presence of four upper molars in this form appears to point to a |