Development of the Mammalian Phylum. 377 



subject, writes as follows as to the persistence of the mar- 

 supial bones in the placental mammals : — " I must here at 

 once observe that I have been unable to discover these in any 

 embryo — and I have examined representatives of all the chief 

 groups — to say nothing of an adult animal." That which 

 persists in the placental mammals is a girdle of cartilage, 

 which in the amphibians and reptiles represents the formative 

 material of the epipubis, and in the marsupials furnishes the 

 marsupial bones which are homologous with this. 



If, therefore, the arguments in favour of the derivation of 

 the placental mammals from the marsupial are untenable, 

 there are, on the other hand, others which tell directly against 

 the theory. The most primitive condition of the brood- 

 apparatus is represented by two so-called mammary pouches, 

 as they are found in the Echidna; the brood-pouch is an 

 acquisition which is to be derived from this, in that the edges 

 of the mammary pouches become completely (temporarily in 

 Echidiui) or partially (in the marsupials) fused together. 



Now the dermal pouches which occur in many ungulates 

 have recently been identified by Klaatsch * as mammary 

 pouches, which he regards as discarded mammary structures, 

 while the remaining pairs of mammary pouches have become 

 completely modified into teats. Klaatsch therefore considers 

 ir to be conceivable that the ungulates never passed through 

 a marsupial stagCj and at any rate concludes that the ungu- 

 lates have never possessed a pouch-structure like the existing 

 marsupials. 



A further weighty objection is to be found in the constitu- 

 tion of the dentition. As I was the first to demonstrate, the 

 dentition of the adult marsupials belongs to the first series, 

 while that of the adult placental mammals represents the 

 second set of teeth. This by itself is at once a deep-seated 

 dift'erence which prevents any homologization. Moreover the 

 marsupial dentition exhibits a type wdiich is firmly closed 

 within itself and from which a further development appears 

 impossible. Quite characteristic is the entrance of a premolar 

 of the second series into the dentition, a peculiarity which 

 has persisted from the Jurassic down to the recent forms. 



To sum up shortly the results of these considerations, we 

 find that tenable arguments for deriving the placental 

 manmials from the marsupials do not exist, but that there are 

 some that tell against such a process. We may well imagine 

 that the placental mammals originated from the ancient 



* Klaatsch, " Ueber Mammartascben bei erwacbsenen Huftieren," 

 Morpb. Jahrbuck, Bd. 18, Heft 2, p. 349. 



Anil. iL May. N. Hist. Ser. 6. Vol. x. 26 



