TRANSACTION'S OF SECTION D. 769 



the small innermost circle. Such ancestors would, properly, not be classed as 

 Mammals, for although they possessed two occipital condyles, yet the absence of 

 mammary glands and the articulation of the mandible with a quadrate bone would 

 necessarily prevent their recognition as such. 



From these ancestral forms, which would probably differ from their nearest 

 vertebrate allies by the presence of two occipital condyles, the retention of the left 

 as the principal aortic arch, the existence of an amnion and the non-existence of 

 branchiae, we may conceive the evolution of the primary Mammalian forms, termed 

 by Huxley Prototheria (indicated in the diagram by the second ring), and of which 

 the evidently highly differentiated species included within the srenera Ornithorhyn- 

 chus and Echidna are the sole existing representatives. 



While such forms by the possession of mammary glands would rank as 

 Mammals, yet the absence of teats, the presence of a deep cloaca, the hypocystic 

 position of the openings of the ureters, and the characters of the generative organs 

 generally, indicate the early stage of their Mammalian organisation. 



It is possible, however, that at this early stage separation into two groups took 

 place (indicated in the diagram by horizontal lines), from one of which proceeded 

 the ancestors of the Placental, and from the other those of the Implacental Mam- 

 mals. That such was the case appears to be indicated by the presence of epipubic 

 bones in both Monotremata and Marsupialia, while their existence, whether in a 

 reduced or rudimentary state, is, to say the least of it, doubtful in all other mammals. 

 Again, a binary division evidently also took place in each of those groups (indi- 

 cated in the diagram by vertical lines), giving origin on one side to the ancestors of 

 the existing Monotremata (shown in the diagram to be a terminal group) and to 

 the Marsupialia, and on the other to those of the Iloniodont and Heterodont sub- 

 classes of the higher Mammalia. 



The next stage of evolution termed by Huxley Metatherial (indicated in 

 the diagram by the third ring), is represented among existing Mammals by the 

 Marsupialia, themselves, like the Monotremata, a terminal group, though there can 

 be no doubt that they are greatly modified representatives of this sub-class. It is 

 more than probable that the now extinct Metatherial ancestors of existing Placental 

 Mammals differed altogether from those of the existing Marsupials, though belong- 

 ing to the same evolutionary stage, in that they possessed neither marsupial bones 

 nor pouches, nor were their young brought forth at a much earlier period than in 

 existing Carnivores and Rodents, but exhibited in common with them a marked 

 advance in development, as evidenced by the presence of mammary teats, in the 

 reduced condition of the. cloaca, in the ento-cystic mode of opening of the ureters, and 

 in the general characters of the generative "organs. Nevertheless these mammals 

 still presented little advance in the intra-uterine mode of nutrition of the foetus, 

 the placenta being still non-allantoic ; indeed, as I am informed by Mr. Sedgwick, 

 the recent discoveries of Mr. Caldwell in Australia, show that in their highly 

 differentiated descendants, the Marsupialia, there is scarcely what deserves°the 

 name of a placenta. 



In the next stage, that of the Eutheria or Higher Mammalia (indicated in the 

 upper semicircle of the diagram by the outer broad ring), we find a great ad- 

 vance in development in the presence of an allantoic placenta which may be non- 

 deciduous, as in the orders Ungulata, Cetacea, Sirenia, and Lemuroidea, and in the 

 Homodont order Scmamata, or deciduate as in the remaining orders ; and this is 

 shown at a glance in the diagram by the position of the words indicating the orders, 

 those with deciduate placenta occupying the outer and higher position in the 

 evolutionary scale. 



It is highly probable that the separation of the Homodont Mammals, represented 

 by the existing and extinct Edentata (which for many reasons should be con- 

 sidered a separate sub-class), took place at an early period of Mammalian evolutkm, 

 perhaps in the Prototherian stage, as I have indicated by the vertical line dividing 

 the upper semicircle into two equal parts. 



The natural affinities of the orders of Heterodont and Homodont Eutherians to 

 one another are indicated, as far as a linear series will allow, by their position in 

 the section to which each belongs. 



1884. 3 d 



