496 Vertebrata. 



it must be remembered that mammilla are absent, and that 

 the mammary glands are primitive in character. The few known 

 forms are edentulous when adult, but horny teeth may be 

 present. Recently it has been demonstrated that the young 

 Ornithorhynchus has true teeth, but that they are lost later. 

 There is no definite external ear. The males have a horny 

 spur on the heel, perforated by the duct of a gland. 



There are only three living species of Monotremes known, they 

 are mentioned below. They are of medium size, and are confined to 

 Australia, New Guinea, and Tasmania. Very little is known 

 definitely concerning the fossil remains of this group. 



1. The Duck-billed Platypus {Omithorynchus paradoxus). Thesnout 

 is flattened, broad, and covered witb a naked skin ; at tbe back of tbe mouth on 

 either side of each jaw, there is a large homy tooth, and a smaller homy ridge in 

 front ; the tail is powerful and flattened ; the feet webbed ; the fur soft. They 

 feed upon small aquatic animals. The eggs are laid two at a time, in holes dug 

 in the ground ; the young ones, when hatched, are fed with milk by the mother. 

 East Australia, Tasmania. ^ 



2. The Spiny Anteater {Echidna aculeata). The snout is narrowed, 

 especially towards the tip, and covered with naked skin ; the mouth small, the 

 tongue long and sticky ; the body covered with hairs and spines ; the tail very 

 short ; strohg digging claws. The food consists of Ants, Termites, etc. The egg 

 (only one is laid at a time) is placed in an unpaired saccular depression on the 

 ventral surface, and there incubated ; the temperature of the sac rises several 

 degrees above that of the body. It serves later to protect the young one, and 

 then atrophies, forming anew before the next oviposition. The brood pouch is 

 absent from Omithorynchus. DifEerent varieties inhabit New G-tiinea, Australia, 

 and Tasmania. The three-toed Echidna {E. [^Proeehidnal Bruijnii), is 

 a near relative ; it has a longer and curved bill, and only three toes on each foot, 

 both hind and fore, whilst E. aculeata has five. New Guinea. 



Order 2. Marsupialia {Marsupials). 



The leading characteristic of this group is the absence of a true 

 placenta ; the outer embryonic membrane does not project as villi 

 into the uterine wall, but the embryo is nourished by a secretion of 

 uterine glands, and is born in a very immature and imperfect 

 condition. Other characters also show the low grade of the 

 Marsupials, as compared with all subsequent orders of Mammalia : 

 for instance, the corpus callosum is feebly developed ; in the female, 

 there is a cloaca, but it is a mere pit, and the two oviducts open 

 separately into the urinogenital sinus. Like the Monotremes, and 

 unlike all other Mammals, the Marsupials have marsupial bones, 

 a pair of peculiar ossifications connected with the pubes and extending 

 forwards in the abdominal wall. They have nothing to do with the 

 marsupium usually present in females. This is an open saccular 

 cavity on the ventral side of the animal, limited by a large fold of 

 gkin : it covers the mammillae^ and the young ones are placed in it 



