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Show 456 MR. M. F. WOODWARD ON MAMMALIAN DENTITION. [May 2, animals. As may be seen (fig. 29) the 3rd incisor (i6) develops in a backward prolongation of the premaxilla situated on the palatal side of the maxilla and side by side with the canine, and only attains its true position in very advanced pouch-specimens. The overcrowded state of the jaw will further explain the complete atrophy of the already reduced 2nd, 3rd, and 5th incisors. And to the same cause w e may, I think, ascribe the abnormal position of these teeth having become displaced by the larger ones till they have assumed a position with regard to the latter which suggests the relation between teeth belonging to the 1st dentition and their successors in the 2nd dentition. . The true reason for the small size of these teeth is probably to be found in their disuse together with an early enlargement of the 1st and 4th incisors. This last character would according to Cope's mechanical theory (2) be enough to account for their entire disappearance; he suggests that the enlargement of any organ causes the abstraction of the growth-energy and material from some adjacent structure, as in the case of the enlarged canines of the Wart-hog causing a reduction of the upper incisors. In trying to account for these reductions in the number of teeth every feature which can possibly bear on the case should be taken into consideration, as it is more probable that a number of factors have acted together to bring about these results, rather than they can be accounted for by one only. It is interesting to note in the light of Cope's suggestion that in this Petrogale, where we find a greater number of these vestigial teeth than in any other Macropid wdiich I have examined (except M. giganteus), the adult incisors are of a much slighter nature than in the latter forms, where some of these vestigial teeth have either completely disappeared or if present have become much reduced. 1 think that the presence of these additional teeth is to be explained rather on the above grounds, than on the supposition that Petrogale penicillata is an extremely primitive form. The presence of the remains of six pairs of upper incisors is altogether unexpected and rather unfortunate, for one would hardly expect to find in so specialized a family as the Macropodidse more incisors than are present in the least specialized Polyprotodonts. In these latter forms with their 5 upper incisors, however, the premaxilla in the foetus is very much crowded, more so than in Macropus. As the teeth in the former all develop at the same time, whereas in the latter, as w e have seen, they develop progressively from before (backwardly), this m a y perhaps accouut for the suppression of the 6th incisor. Judging from the condition of the adult in Perameles, I should imagine that the first incisor to disappear was number 5, as there is a large diastema in the adult between the 4th and 5th incisors. This suggestion was further strengthened by the study of a series of sections through the jaw of a young Perameles, while the premaxilla was still short aud consequently crowded with teeth : at this stage a slight but definite thickening of the dental lamina (fig. 28) was observable between |