454 ZOOLOGY. 



this also the Monotremata are reptilian, having complete 

 coracoids. In the skull the articulation of the mandible 

 with the squamosal, and the conversion of the articular and 

 quadrate bones into the malleus and incus of the tympanic 

 cavity, are peculiarities found in all Mammals and in no 

 other Vertebrates. The backward development of the 

 cerebral hemispheres, the corpus callosum, the corpora 

 quadrigemina, and the spirally coiled cochlea, are characters 

 of the brain and sense organs. In the reproductive system 

 the cloaca is divided into a dorsal passage terminating 

 the rectum, and a ventral connected with the bladder, and 

 forming the urinogenital duct ; the Monotremata, how- 

 ever, have an undivided cloaca like the Sauropsida. In the 

 latter also the eggs are rather large, provided with yolk, 

 and are laid and hatched afterwards : in other Mammals 

 the egg develops in the uterus, and the young are born 

 alive ; the diagnostic character is, however, not the latter 

 fact but the formation of the placenta by the union of the 

 allantois with the uterine wall. The true allantoic placenta 

 is only fully developed in the higher Mammals ; in Mar- 

 supials, with the exception of one, Perameles, the yolk sac 

 alone adheres to the walls of the uterus and forms what is 

 called an umbilical placenta. In accordance with these 

 important differences Mammals are divided into three sub- 

 classes : the Prototheria or Monotremata, containing only 

 Ornithirhynchus and Echidna; the Metatheria or Mar- 

 supials ; and the Eutheria, including the rest of the 

 class. 



