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Show 1880.] ARRANGEMENT OF THE MAMMALIA. 655 dentition of which only a vestige is left in the Marsupialia. Among existing Rodents, in fact, all conditions of the milk-dentition exist, from a number equal to that of the permanent incisors and premolars (as in the Rabbit1) to none at all. The same thing is observed in the Insectivora, where the Hedgehog, and probably Centetes, have a full set of milk-teeth, while none have yet been found in the Shrews. In these cases it is obvious that the milk-dentition has gradually been suppressed in the more modified forms ; and I think that there can be no reasonable doubt that the existing Marsupials have undergone a like suppression of the deciduous teeth, in the course of their derivation from ancestors which possessed a full set. Again, no existing Marsupial possesses an unmodified pentadactyle pes. If the hallux is present, it presents an extensive movement in adduction and abduction; in fact, the pes is prehensile. This is the case in the Phascolomyidee, Phalangistidce, Phascolarctidee, and Didelphidce. The Dasyuridee present the same type of pes, with the hallux reduced or suppressed. Hence, considering the relations of the Macropodidee and the Peramelidee with the Phalangers, it seems likely that the hind foot in these groups is also a reduced prehensile pes ; in which case this special modification of the foot would characterize the whole of the existing Marsupialia. Thirdly, the most marked peculiarities of the reproductive organs and processes iii the Marsupial are in no wise transitional, but are singularly specialized characters. The suspension of the scrotum in front of the root of the penis is unlike any arrangement in the higher mammals ; and the development of the bulb and of Cowper's glands is in excess of any thing observable in them. In the female, the cystic urethra is as completely separated from the vagina as it is in the higher mammals; while the doubling of the vagina must, in my opinion, also be considered a special peculiarity which leads from, rather than towards, the higher mammals. In a Monotreme, in fact, the anterior end of the genito-urinary passage exhibits two very short dilatations or cornua, one on each side. In the middle line, a little distance behind these, the ureters open on a prominent ridge-like papilla. The opening of the bladder lies in front of and below the genital cornua. Now, if we compare this arrangement with that which obtains in the lower forms of the higher Mammalia, we find that the ureteric papilise have separated laterally and moved forwards, in such a manner as to occupy the base of the bladder, and the genital cornua come to lie behind and somewhat dorsad of them. At the same time a longitudinal separation has taken place between what may be called the " ureteric " region of the 1 The deciduous molars and the posterior deciduous upper incisors of the Babbit have long been known. But I have recently found that unborn Babbits possess, in addition, two anterior upper and two lower deciduous incisors. Both are simple conical teeth, the sacs of which are merely embedded in the gum. The upper is not more than one hundredth of an inch long, the lower rather larger. It would be interesting to examine fcetal Guinea-pigs in relation to this point; at present they are known to possess only the hindmost deciduous molars, and, so far, agree with the Marsupials. |