xiii PHYLUM CHORDATA 449 



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 (except 

 in some Marsupials) 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 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. 

 When a placenta is 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 (Didelphyidoe), the Dasyures 



(Dasyuridw), 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 

 incisors large, the canines usually small or absent; the molars 

 blunt, with tubercles or transverse ridges. 



This order includes the Wombats (Phascolomyidce), the Phalan- 

 gers (Phalangeridce), and the Kangaroos (Macropodidce). 



SECTION B. EUTHERIA. 



Theria in which a marsupium is absent, and the young are always 

 nourished in utero, for a relatively considerable period, through the 

 agency of a placenta. The anus and urinogenital aperture are 

 not surrounded by a common sphincter. The alisphenoid never 

 contributes to the formation of the wall of the tympanic cavity : 

 except in the thyrocoidea and some Rodents, the jugal takes no 

 part in bounding the glenoid cavity, and there are no marsupial 

 bones. A corpus callosum is present. 



ORDER 1. EDENTATA. 



Eutheria, in which the teeth are absent in the adult or the 

 ^dentition is imperfect, incisors and canines being seldom repre- 



VOL. II G G 



