525 



There are about 2,300 living and over 3,000 extinct species of 

 mammalia known at the present time. Many of the extinct 

 groups are very imperfectly known, and it seems advisable in 

 the present state of knowledge to give some of these the rank 

 of orders. 



Order 1. MONOTREMATA.* 



(Sometimes called Ornithodelphia and Prototheria.) 



Oviparous Mammalia with merohlastic ova. 



The monotremes differ from all other mammals in possessing 

 the following characters, some of which may be described as 

 reptihan. The mammary glands are without teats, and the 

 brain is without a corpus callosum. The pectoral girdle has a 

 large coracoid whicli reaches the sternum, and a precoracoid. 

 There is also a large T-shaped interclavicle. The vertebrae 

 are without epiphyses, and the ribs are provided only with a 

 capitulum. Marsupial bones are present. The right auriculo- 

 ventricular valve is incomplete and partly muscular. The 

 testes retain their abdominal position. The ureters do not 

 open into the bladder but into a urinogenital sinus, which com- 

 municates with the alimentary canal, so that there is a cloaca. 

 The eggs are large and meroblastic, and are laid at an early 

 stage of development. They are however undoubtedly mam- 

 mals, being provided with hairs, warm blood, non-nucleated 

 red blood-corpuscles and a left aortic arch. 



There are but three living genera, and very few fossils are 

 known. The living forms are confined to Austraha, Tasmania, 

 and New Guinea. 



The form of the body and the mode of life partly recall the 

 anteaters and hedgehog {Echidna, Fig. 273) and partly the 



* "Monotremen u. Marsupialen " in Semon's Zoologische Forschungs- 

 reise in Australien, etc. Bd. 2, 1897. R. Owen, " Monotrcmata" in Todd's 

 Enciiclopaedia of Anatomy and Physiology, 1847. W. H. Caldwell. The 

 Embryology of Monotrcmata and Marsupialia, Phil. Trans. 178, 1887, 

 p. 463. 



