254 MAMMALIA. 



ORDER 2. MONOTREMATA. 



The only remains of extinct monotremes hitherto discovered 

 are a few isolated limb-bones, one atlas vertebra, and one im- 

 perfect skull from the superficial deposits of Queensland and 

 New South Wales. A gigantic humerus from the Pliocene of 

 Gulgong, New South Wales, is named Ornithorhynchus maxi- 

 mus. A very small tibia and mandible, from the Pleistocene of 

 Queensland, are described as 0. agilis. The imperfect skull and 

 atlas vertebra, from the Pliocene of Gulgong, are relatively very 

 large and referred to Echidna, under the name of Echidna 

 (Proechidna} robusta. Echidna (Proechidna) oweni is a species 

 based on limb-bones of another very large form from the 

 Pleistocene of the Wellington Caves, New South Wales. 



Sub-Class 2. Metatheria. 



There are many difficulties in interpreting the characters 

 of the Metatherian mammals ; and as Paleontology furnishes 

 only skeletons for examination, it cannot contribute much 

 towards the solution of the problems, which are chiefly con- 

 nected with the organs of reproduction. Modern research, 

 however, suggests that the marsupials, which are the sole 

 surviving representatives of the Metatheria, are descended 

 from mammals which possessed at least a rudimentary discoid 

 allantoic placenta. If so, they have become non-placental by 

 degeneration. It is therefore noteworthy that the earliest 

 known complete mammalian skeletons from the Lower Eocene 

 formations, which pass upwards by insensible gradations into 

 undoubted Eutheria, are scarcely distinguishable from the 

 skeletons of the more generalized existing marsupials (e.g., 

 Thylacinus). These ancestral types may thus have been 

 essentially placental, and retained that character in .their main 

 line of development, while losing it in the less vigorous line 

 which now survives only in the Australian region and tropical 

 America. There are indeed marks of degeneration in the 

 marsupial skeleton itself; for the lower vertebrates demon- 

 strate clearly that the loss of successional teeth is to be 

 interpreted as such, and no existing marsupial is known to 



