20 



MAMMALIA 



vertebrates. In no other division of the animal kingdom have we as much 

 material at our disposal for the reconstruction of the genealogical succession. 



Concerning the origin of the Mammalia, birds and modern types of fishes 

 cannot be considered as standing in the direct line of descent. The Amphibia, 

 too, from which Huxley thought the Mammalia are descended, are more distantly 

 removed than the Reptilia. Among these the Theriodontia are so like the 

 mammals that Owen and Cope considered them as true ancestors. According 

 to the latest researches of Broom, Watson, and others, it is a well-established 

 fact that the mammals are closely related to or derived from the theriodonts. 



The Mammalia are commonly divided into two subclasses, depending 

 upon the nature of their embryological development ; namely, the Eplacentalia 

 and the Placentalia. The former include the orders Monotremata and Marsupialia ; 

 the latter, the orders Insectivora (including Tillodontia), Chiropttera, Carnivora, 

 Oetacea, Edentata, Bodentia, Ungitlata, Notoungulata, ■ Suhungidata (including 

 Sirenia) and the Primates. 



Subclass A. EPLACENTALIA. 



Embryonic development luithout placenta. 



Order I. MONOTREMATA ^ Bonaparte. 



( Ornithodelphia Blain ville. ) 



Pectoral arch with separate coiucoid, frecoracoid and interclavicle. Scapula 

 without prespinous portion of blade. Pelvis with marsupicd bones. Jaw extended, 

 edentulous or with vestigial teeth. Oviparous. Mammary glands loithout nipples, 

 opening into separate glanckdar areas. Urethra and genital passages opening into 

 the cloaca, which leads into the lower extremity of the rectum. Body-temperature 

 not similar everyiohere. 



This order is of interest on account of its many reptilian features and also 

 because of its high degree of specialisation. It is, however, of little importance in 



Fig. -25. 

 Lower left molars of the young of Ornithorhynchus. 5/o. (After O. Thomas.) 



palaeontology, for the few definitely known remains are found in the Pleistocene 

 of Australia and belong to the three recent genera, Ornithoi-hynchus (Fig. 25), 

 Echidna (Tachyglossus) and Proechidna. 



^ G-regory, W. K., The orders of mammals. Bull. Amer. Mus. Nat. Hist., vol. xxvii., 1909, 

 pp. 144-162. — Thomas, 0., Dentition of Ornithorhynchus. Proc. Roy. Soc. Loud., vol. xlvi., 1889. 

 — Van Bemmeleyi, J. F., Der Schiidelbau der Monotremeu. (In Semon, Zoologische Forscliuugsreisen 

 in Australieu. Jena, 1901.) 



