xiii PHYLUM CHORDATA 465 



X 



ORDER 2. DIPROTODONTIA. 



Marsupials with not more than three incisors on each side in 

 the upper jaw, and usually only one in the lower ; the central 

 incisors large, the canines usually small or absent ; the molars 

 blunt, with tubercles or transverse ridges. 



This order includes the Wombats (Phascolomyidce), the Phalan- 

 gers (Phalangeridce), and the Kangaroos (Macropodidce). 



SECTION B. EUTHERIA. 



Theria in which a marsupium is absent, and the young are always 

 nourished in utero, for a considerable period, through the agency 

 of a placenta. The anus and urinogenital aperture are usually 

 not surrounded by a common sphincter. The alisphenoid never 

 contributes to the formation of the wall of the tympanic cavity ; 

 except in the Hyracoidea and some Rodents, the jugal takes no 

 part in bounding the glenoid cavity ; and there are no marsupial 

 bones. A corpus callosum is present. 



ORDER 1. EDENTATA. 



Eutheria in which the teeth are absent in the adult or the den- 

 tition is imperfect, incisors and canines being seldom represented ; 

 and, though there may be numerous premolars and molars, these 

 never form roots and are devoid of enamel. All, with the exception 

 of two genera, are monophyodont. The sacral vertebrae are fre- 

 quently in excess of the number usual in other orders. The coracoid 

 process is usually relatively larger than in other Eutheria, and 

 does not become completely fused with the scapula. The brain 

 is sometimes of low, sometimes of comparatively high organisation. 

 The placenta is deciduate or non-deciduate, diffuse, zonary or 

 discoidal (vide p. 576). 



There are five families usually comprised in the order, each 

 characterised by the presence of a number of remarkable and peculiar 

 features : viz., the Sloths (Brady podidce), the American Ant-eaters 

 (Myrmecophagidce), the Armadillos (Dasypodidce), the Scaly Ant- 

 eaters (Manidce), and the Cape Ant-eaters (Orycteropodidce). The 

 two last are probably not sufficiently nearly related to the rest 

 to be included with them in one order. 



ORDER 2. CETACEA. 



Aquatic Eutheria with large head, fish-like fusiform body, 

 devoid of hairy covering, with the pectoral limbs paddle-like, the 

 pelvic limbs absent, and with a horizontal caudal fin. A vertical 

 dorsal fin is usually present. There is a long snout, and the 

 nostrils open by two lateral external apertures or a single median 

 one situated in all the recent forms far back towards the summit 



