464 ZOOLOGY SECT. 



Sub-class I. Prototheria. 



Mammals in which the mammary glands are devoid of teats ; 

 the oviducts are distinct throughout, and there is a cloaca into 

 which the ureters and urinary bladder open separately. In the 

 centra of the vertebrae 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 develop- 

 ment, enclosed in a tough shell. 



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

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

 Spiny Ant-eater (Echidna). 



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 ; 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 com- 

 pletely 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, 

 and the early development of the young takes place in the uterus. 



SECTION A. METATHERIA (MARSUPIALIA). 



Theria in which the young, born in a comparatively rudimentary 

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

 tegumentary pouch the marsupium. 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 (Didelphyidce), the Dasyures 

 (Dasyuridce), and the Bandicoots (Peramelidce). 



