454 GENERAL ZOOLOGY 



SUBCLASS 1. ARCHAEORNITHES. Extinct birds. 

 Example : A rchaeoptcryx. 



SUBCLASS 2. NEORNITHES. Includes four orders of 

 extinct birds as well as all existing birds. 



CLASS VII. MAMMALIA. Mammals. A mammal is a 

 homoiothermous animal, covered more or less with hair and 

 provided with mammary glands on the ventral surface of the 

 body which in the case of the female are used in suckling the 

 young. The teeth are thecodont and are of four kinds: 

 incisors, canines, premolars, and molars. Except in certain 

 aquatic mammals, an external ear, in addition to a middle and 

 inner ear, is present. The middle ear contains three ossicles: 

 malleus, incus, and stapes. A muscular diaphragm divides the 

 body cavity into a thoracic cavity containing the heart and 

 lungs in front and an abdominal cavity containing the rest of 

 the viscera behind. Each lung lies freely in a pleural cavity. 

 The heart consists of two ventricles and two atria, and the 

 circulation is completely double. The red corpuscles of the 

 blood lack nuclei. 



SUBCLASS 1. PROTOTHERIA. Egg-laying mammals. 

 These have a single cloacal opening for both the urinogenital 

 and alimentary systems. The mammary glands have no 

 distinct nipples. The egg is large, contains much yolk, and 

 resembles the reptilian or avian egg. As in birds, the left 

 ovary and left oviduct only are functional. 



Examples: Ornithorhynchus anatinus, the Australian duck- 

 bill or platypus, is about 20 in. in length. It has a duck- 

 like bill, webbed feet, and is covered with fur. Echidna 

 aculeata, the spiny ant eater, also found in Australia. These 

 with a few other species are the only representatives of the 

 entire subclass. They occur only in Australia and the neigh- 

 boring islands. 



SUBCLASS 2. METATHERIA. Marsupials. These are 

 viviparous mammals whose young are born in an immature 

 state and are usually carried for a period after birth in a 

 marsupium or pouch, located on the abdomen of the mother. 

 A cloaca is absent and the urinogenital and anal openings are 

 distinct. The mammary glands are within the marsupium and 

 are provided with teats. The vagina is partially or completely 

 double. 



