154 Transactions of the Society. 



incisors, the molars and premolars being tubercular. Numerous 

 remains of these small mammals have been met with in this 

 country, in America, and in France, the earliest being the Droma- 

 thcrium sylvestre, from the Trias of North Carolina. 



Monotremata. — The lowest type of living mammals (the Mono- 

 tremata) are oviparous, the egg being apparently placed by the 

 female in the marsupium or pouch of the mother, the young re- 

 maining attached to the parent until able to feed themselves. 



Metatheria: Marsupialia. — In the Marsupialia, which com- 

 prise the kangaroos and wombats, the young is not enclosed in an 

 egg at birth, but is produced as a very minute and immature foetus, 

 and placed by the parent in the marsupial pouch, where it becomes 

 attached to the mammary gland, and is carried in this receptacle 

 until able to run alone. 



The kangaroos and wombats are almost entirely confined at 

 the present day to Australia, but one genus, Didelphys, is found 

 living in South America, while fossil remains occur in Tertiary 

 deposits in Europe. Possibly some of the small extinct mammals, 

 whose remains have been found in the Purbeck and Stonesfield 

 Slate, may have belonged to the Marsupialia. 



In Tertiary times, animals of very large size, such as the 

 Diprotodon, the Nototherium, and Thylacoleo existed in numbers 

 upon the Australian continent ; but these are all now extinct, and 

 only the existing Kangaroos, the small Wombats, and Opossums, 

 survive. 



Eutheria : Placental Mammals. — The origin of the two 

 groups of marine placental mammals, the Cetacea and Sircnia, 

 still remains uncertain, and Palaeontology does not afford us any 

 information thereon. 



Cetacea. — The largest of all living or extinct animals belong to 

 the whale tribe, probably the great Eight-whale, measured not short 

 of 100 ft. in length and was many tons in weight ; the Cetacea are 

 all warm-blooded mammals, and have probably been derived from 

 Terrestrial ancestors who at some distant period took up an aquatic 

 existence probably within the tropics ; the body in these animals 

 is not clothed in fur, but beneath the skin is a thick layer of fat 

 ("blubber") which as effectually protects the vital organs from the 

 cold in its watery home, as does the fur of any arctic animal on the 

 land. The remains of Cetacea, particularly of the Toothed Whales, 

 the Sperm Whale, the Dolphin, etc., are met with in deposits of 

 later Tertiary age, such as the Crag of Suffolk and of Antwerp. 



The earliest known Cetacea (Zevglodon) were provided with 

 cheek teeth with double fangs ; whereas the later Cetacea have no 

 distinction in the teeth in their jaws, which are all simple one- 

 fanged teeth of the same pattern. In the Eight-whales, teeth, 

 except in the foetus, are unknown, their place being taken by 

 horny plates of whalebone known as baleen, which differs greatly 



