Xii PHYLUM CHORDATA 525 



veloped ; the bones of the skull early coalesce by the oblit- 

 eration of the sutures ; there is a large coracoid articulating 

 with the sternum, and a T-shaped episternum, and there is a 

 pair of epipubic (marsupial) bones. In the brain a corpus 

 callosum is absent. The ova are discharged in an early stage 

 of their development, enclosed in a tough shell. 



This sub-class comprises a single living order, the Mono- 

 tremata, including the duck-bill or Platypus (Ornithorhyn- 

 chus} and spiny ant-eater {Echidna). 



The Theria are mammals in which the mammary glands 

 are provided with teats ; the oviducts are united in a longer 

 or shorter part of their extent, and there is no cloaca, the 

 ureters opening into the base of the bladder. The centra of 

 the vertebrae possess distinct epiphyses ; the bones of the 

 skull in most instances do not completely coalesce, most of 

 the sutures remaining distinguishable throughout life ; the 

 coracord is represented by vestiges, and an episternum is 

 absent as a distinct bone. The early development of the 

 young takes place in the uterus. 



Of the Theria again there are two sections, the Metatheria 

 or Marsupialia and the Eutheria. 



The section Metatheria comprises all the pouch-bearing 

 mammals or marsupials, such as the opossums, the dasyures, 

 the bandicoots, the wombats, the phalangers, and the kan- 

 garoo, nearly all, with the exception of the opossums, con- 

 fined to the Australian region. They are characterised by 

 the possession of a pouch or marsupium, within which the 

 young, born in rudimentary and helpless condition, are 

 sheltered. They also possess a pair of peculiar bones, the 

 epipubic or marsupial bones (present also in the Prototheria) , 

 attached to the pubes. 



In the Eutheria marsupium and marsupial bones are 

 absent. This section comprises the great majority of 



