370 MAMMALIA [CLASSIFICATION. The first class of variations lias given rise to a distinc tion into two principal kinds of placenta (1) simple or non-deciduate, and (2) deciduate. In the former the foetal villi are received into corresponding depressions of the maternal surface, from which at the period of parturition they are simply withdrawn. In the second or more complex form the relation is more intimate, a layer of greater or less thickness of the lining membrane of the uterus, called- " decidua," becoming so intimately blended with the chorion as to form part of the placenta proper, or that structure which is cast off as a solid body at parturi tion. In other words, in the one case the line of separation between the placenta and uterus at birth takes place at the junction of the foetal and maternal structures, in the other through the latter, so that a portion of them, often of considerable thickness, and containing highly organized structures, is cast off with the former. It has been thought that the distinction between these two forms of placentation is so important as to constitute a sufficiently valid basis for a primary division of the placental mammals into two groups. It has, however, been shown that the distinction is one rather of degree than of kind, as inter mediate conditions may exist, and it is not improbable that in different primary groups the simpler, non-deciduate form may have become developed independently into one or other of the more complex kinds. Apart from its intimate structure, the placenta may be met with of very varied general form. It may consist of villi scattered more or less regularly over the greater part of the surface of the chorion, the two extremities or poles being usually more or less bare. This form is called the "diffused placenta." It is probably a primitive condition, from which most of the others are derived, although its existence must presuppose the absence of the umbilical vesicle as a constituent of the chorionic wall. It is found at present in the Manis among Edentates, the Cetacea, Sirenia, the Perissodactyle Ungulates, and the Camels, Pigs, and Chevrotains among the Artiodactyles. Such placentae are always non-deciduate. In the true Ruminants or Pecora, among the Artiodactyle Ungulates, the villi are aggregated in masses called cotyledons, with bare spaces between. Such a placentation is called "polycotyledonary." In another modification the villi are collected in a more or less broad band encircling the chorion, leaving a very large portion of the two poles bare, constituting the "zonary placenta," characteristic of the Carnivora, and also occur ring in the Elephant, Hyrax, and Oryderopus. The fact of the form of the placenta of these three last-named animals agreeing together, and with that of the Carni vora, does not, however, necessitate the ascription of zoological affinities, as the placenta of the Carnivora has been shown to be at first discoidal, and to become zonary by spreading round the chorion in the course of develop ment. In the other cases, although it must be admitted that the early stages have not been well observed, it is quite probable that it may be derived from a diffused placenta, in which the foetal villi have disappeared from a larger space than usual of the two poles of the ovum. In another form one pole only of the chorion is non- vascular, the placenta assuming a dome or bell shape, as in the Lemurs and the Sloths. The transition from this, by the gradual restriction of the vascular area, is easy to the oval or discoidal form of placenta of the Anteaters, Armadillos, and Primates. The discoidal placenta of the Rodents, Insectivores, and Chiroptera, though showing so much superficial resemblance to that of the last-named order as to have caused them formerly to be associated in one primary group, is now known to be developed in another manner, not by the concentration of villi from a diffused to a limited area, but by retaining the area to which it was originally restricted in consequence of the large surface of chorion occupied, as before mentioned, by the umbilical vesicle. To compensate for the smallness of area, the complex or deciduate structure has been developed. We may conclude that, although the characters and arrange ment of the foatal structures may not have that extreme importance which has been attributed to them by some zoologists, they will form, especially when more completely understood, valuable aids in the study of the natural affinities and evolution of the Mammalia. 1 CLASSIFICATION OF THE MAMMALIA. As stated at the commencement of this article, the mammalian class, as at present known either by existing or extinct forms, is completely isolated from all other groups of the animal kingdom, but it is impossible to refrain from speculating as to its origin and nearest affinities. In arranging the classes of vertebrates in a linear series it is customary to place them in the following order Pisces, Amphibia, Keptilia, Aves, Mammalia, an order which may possibly indicate the relative degree of elevation to which the most completely developed members of each class attain, though it would be a great mistake to suppose that such an arrangement expresses the true relationship of one to the other, and still less must it be imagined that in the process of evolution any of the higher classes are necessarily derived directly from those nearest below them in this serial arrangement. On the contrary, some argu ments recently set forth by Professor Huxley 2 point very strongly to the conclusion that, in looking among verte brates for the progenitors of the Mammalia, we must pass over all known forms of birds and reptiles, and go straight down to the Amphibia. In addition to the characters derived from the conformation of the pelvis upon which the argument is primarily based, the following reasons are given for this conclusion : "The Amphibia are the only air-breathing Vertebrata which, like mammals, have a dicondylian skull. It is only in them that the articular element of the mandibular arch remains cartilaginous, while the quadrate ossification is small, and the squamosal extends down over it to the osseous elements of the mandible, thus affording an easy transition to the mam malian condition of those parts. The pectoral arch of the Monotremes is as much amphibian as it is sauropsidian ; the carpus and the tarsus of all Sauropsida, except the Chelonia, are modified away from the Urodele type, while those of the mammal are directly reducible to it. Finally, the fact that in all Sauropsida it is a right aortic arch which is the main conduit of arterial blood leaving the heart, while in mammals it is a left aortic arch which per forms this office, is a great stumbling-block in the way of the derivation of the Mammalia from any of the Saur opsida. But, if we suppose the earliest forms of both the Mammalia and the Sauropsida to have had a common Amphibian origin, there is no difficulty in the supposition that, from the first, it was a left aortic arch in the one series, and the corresponding right aortic arch in the other, which became the predominant feeder of the arterial system." There is so much in common between the very aberrant Monotremes, upon the structure of which the above con clusions are mainly based, and all other known mammals that we cannot but suppose they are derived, perhaps at some remote period, from one stock, some of the pre dominating characters of which survive in the existing 1 For a full exposition of the present state of knowledge on tins subiect, see the various memoirs of Professor Turner, and especially F. M. Balfour s Treatise on Comparative Embryology, 1881, vol. ii. 2 Proceedings of the Royal Society of London, vol. xxviii., 1879,
p. 395.