478 ZOOLOGY SECT 



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 callosuin 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 (Ornithorhynchus) and the 

 Spiny Anteater (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, 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 (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 

 (Dasi/uridce), and the Bandicoots (Peramelidce). 



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 



