vi POSITION OF MARSUPIALS 119 



in Jlyrmecobius (alone) is the third toe the longest ; and no great 

 difference can be detected between the third and fourth toes in 

 the case of the genera Phascologcde, Didelphys, and some others. 

 Professor Leche compares the predominance of the fourth toe 

 with the hyperphalangeal condition in the fourth toe of the embryo 

 Crocodile, and considers it an archaic feature, not surpassed by 

 the ancient characteristics of the Monotremata. Again it has been 

 pointed out that in Phascologale and Perameles, the epistropheus 

 (axis vertebra) has a separate rib as in Ornithorhynclius. In the 

 third place, the likeness of the teeth of Myrmecobius to those of 

 Ornithorliynchus is an argument in the same direction, which is 

 furthermore supported by the great age (Mesozoic) of the Meta- 

 therian group, if we are right in regarding those extinct creatures 

 as Marsupials. 



AVe may now mention certain facts which are not so generally 

 used. The partly primitive structure of the right auriculo- 

 ventricular valve in the Monotremata has no counterpart in 

 any Marsupial which has been dissected ; but there are traces 

 in the latter of the characteristic " ventral mesentery " of Orni- 

 thorhynchus and Echidna,} Mr. Caldwell's interesting observation 

 upon the segmenting egg of the Marsupial, the incompleteness 

 of the first segmentation furrow (reminding us of the meroblastic 

 ovum of the Mouotreme), may possibly not turn out to be so 

 exclusively Marsupial a feature as has been thought. 



The balance of evidence thus points to the nearer relationship 

 of the Marsupials to the Eutherian mammals ; and their great 

 specialisation combined with certain evidences of degeneration 

 (disappearance in part of the milk dentition), and their age, point 

 to the fact that they are, at any rate, the descendants of an early 

 form of Eutherian. But they must have separated from the 

 Eutherian stock after it had acquired a definite diphyodonty and 

 the allantoic placenta, the two principal features of the Eutherian 

 as opposed to the Prototherian mammals. 



Nevertheless it seems probable that the Marsupial tribe is 

 derived from some of the earliest Eutherians. And on this view 

 may be explained the retention of Prototherian characters. 



The remaining Eutheria are obviously all to be referred to 

 one great division with the possible exception of the Whales, 

 whose affinities form one of the principal difficulties to the student 

 1 In Dendrolagus at any rate. See Proc. Zool. Soc. 1895, p. 132. 



