TRANSACTIONS OF SECTION D. 



769 



' I 



the small innemiost circle. Siicli ancestors would, properly, not Ixs classed as 

 Mammals, for altboufrli they possessed two occipital condyles, yot, the absi'iice of 

 inammarv gl'iiids and the articulation of the inaudible w'tli a quadrate bone would 

 iiece^siui'iv prevent their recof^nition as such. 



From these ancestral forms, wbich would probably ditfer from their nearest 

 vertebniti' allies by the presence of two occipital condyles, the retention of the left 

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

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

 livlliixley Prototheria (indicated in thediag-ram bytl;e second ring), and of which 

 the evidoiilly highly differentiated species included within the irenera Ornithorhyu- 

 fliii3 ami l*]chidna are the sole existing representatives. 



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

 )!;immals, yet the absence of teats, the presence of a deep cloaca, the hypocystic 

 iisition of the openings of the ureters, and the characters of the generative (n-gans 

 irencrallv, indicate the early stage of their Mammalian organisation. 



It is possible, however, that at this early stage separation into two gi-oups took 

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

 the ancestors of the Placental, and from the other those of the Im])lacental Mam- 

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

 Ijdiii'S 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 tlie diagram by vertical lines), giving origin on one side to the ancestors of 

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

 the Marsupialia, and on the other to tb.ose of the Hoinddont and Ileterodout sub- 

 clafsesof tlie liigher Mammalia. 



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

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

 Marsu])ialia, themselves, like the Monotremata, a terminal group, though there can 

 he 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 dilVer«'d altogether from those of the existing Marsupials, though belong- 

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

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

 existing ( 'arnivorcs and Rodents, but exliibited in common with them a marked 

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

 reduced condition ofthe cloaca, in the cnto-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, 

 tlie 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, Oetacea, Sirenia, and Lemuroidea, and in the 

 Homodont order Squamata, 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 evolution, 

 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 iu 

 the section to which each belongs. 

 1884. 3 D 



