478 ZOOLOGY sbct 



centra of the vertebrifi the epiphyses are absent or imperfectly 

 developed ; the bones of the skull early coalesce by the oblitera- 

 tion of the sutures ; there is a large coracoid articulating with 

 the sternum, a T-shaped episternum, and a pair of epipubic 

 (marsupial) bones. A corpus callosum is absent. The ova are 

 meroblastic, and are discharged in an early stage of their 

 development, enclosed in a tough shell. 



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

 including the Duck-Bill or Platypus (Ornithorhynclius) and the 

 Spiny Anteater (Uchidna). 



Sub-class II. — Theria. 



Mammals in which the mammary glands are provided with 

 teats ; the oviducts are united in a longer or shorter part of their 

 extent, and open into a urinogenital canal ; there is usually no 

 trace of a cloaca ; the ureters open 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 

 coracoid is represented by vestiges, and an episternum is absent 

 as a distinct bone. The ova are holoblastic (except in some 

 Marsupials), and the early development of the young takes place 

 in the uterus. 



Section A. — Metatheria (Maesupialia). 



Theria in which the young, born in a comparatively rudimentary 

 condition, are sheltered during their later development in an in- 

 tegumentary pouch — the marsupiimi. A common sphincter muscle 

 surrounds anus and urinogenital aperture : the vaginae are distinct. 

 The tympanic cavity is partly bounded by the alisphenoid ; the 

 jugal furnishes a part of the glenoid cavity for the condyle of the 

 mandible ; there are well-developed epipubic bones. There is no 

 corpus callosum. A placenta is usually wanting ; when present, it 

 is functional only for a short period. 



Order 1. — Polyprotodontia. 



Marsupials with numerous, small, sub-equal incisor teeth, and 

 large canines ; the molars provided with sharp cusps. 



This order includes the Opossums {Didelphyidm), the Dasyures 

 (BasyuridcB), and the Bandicoots {Peramelidai). 



Order 2.— Diprotodontia. 



Marsupials with not more than three incisors on each side 

 in the upper jaw, and usually only one in the lower ; the central 



