Characteristics of the Vertebrata. xlv 



some Mammalsj such as the true Cetaeea, which have only a single 

 set of teeth, the pulp atrophies or underg-oes calcification ; and a 

 term is thus necessarily put to the duration of the teeth, and of the 

 life of the animal. In others, wliich are similarly ' Monophydont/ 

 as the Sloths {Bradyiwda), amongst the Bruta, the pulp is persistent ; 

 as it is also in the ' Diphyodont ' Armadillos belonging to the same 

 order, and in the incisors of all, in the molars of some Rodents, and 

 in the permanent incisors, or ' tusks ^ of the Elephant. It is only in 

 the Mammalian class that teeth have been observed to be implanted 

 by more than a single fang, and the dentinal tissue is ordinarily 

 free from anchylosis with the alveolus in which the tooth is lodged. 

 The digestive tract is always rich in interstitially-placed glands, 

 and of the larger glands appended to it by ducts, none are ever 

 wanting except the oral salivary glands in the true Cetaeea, and 

 one pair of these glands, the parotid, in the Monotrematous 

 Eehidna. There is much variety from order to order, as to the 

 simplicity and comjilexity of the stomach, and as to the presence or 

 absence of intestinal coeca. It is only in the Ornithodelphia, thence 

 called ' Monotremata,' that the generative and renal ducts are 

 confluent for any great distance with the terminal segment of the 

 intestine, so as to form a true ' cloaca •' though in Marsupialia, 

 as also in certain Rodeniia, in Centetes amongst the Insectlvora, 

 and in certain Bruta, a common sphincter muscle may surround 

 the distal orifices both of urogenital and of the rectal tubes. 



The red-blood corpuscles of Mammalia difier from those of all 

 other Vertebrata not only in being ' apyrenaematous,' but also in 

 being, with the exception of those of the Camelidae, circular. Their 

 heart is always quadrilocular ; their aorta always single, and bent 

 over the left bronchus. The valves, which in other Vertebrata 

 guard the entrance of the great veins into the right auricle, are 

 either absent as usual, or rudimentary as in Bradfpns, Elephas, 

 Simiadae. Correlated with this structural arrangement is the 

 fact that in Mammals the ventricles are the first, the auricles 

 the second in point of time to contract in each systole. In many 

 Rodentia and Insectlvora, and in all Marsupialia and Montremata, 

 there are two superior venae cavae. The lymphatic and lacteal 

 glands are always largely developed, as are also the tonsillar and 

 Peyerian aggregations of adenoid substance in the walls of the 

 digestive tube. The entrance to the larynx is always protected 



