or THE CLASS MAMMALIA. 21 



guide to a conception of the most natural primary groups of 

 the class Mammalia, I next proceed to define the groups of se- 

 condary importance, or the subdivisions of the foregoing sub- 

 classes. 



In the Lyencephalous Mammalia some have the * optic lobes' 

 simple, others partly subdivided, or complicated by accessory 

 ganglions, whence they are called ' bigeminal bodies.' The 

 Lyencephala with simple optic lobes are 'edentulous' or without 

 calcified teeth, are devoid of external ears, scrotum, nipples, 

 and marsupial pouch : they are true ' testiconda ;' they have 

 a coracoid bone extending from the scapula to the sternum, and 

 also an epicoracoid and episternum, as in Lizards ; they are un- 

 guiculate and pentadactyle, with a supplementary tarsal bone 

 supporting a perforated spur in the male. The order so charac- 

 terized is called ' Monotremata,' in reference to the single excre- 

 tory and generative outlet, which, however, is by no means pecu- 

 liar to them among Mammalia. The Monotremes are insectivo- 

 rous, and are strictly limited to Australia and Tasmania. 



The MABSUPiALiAare Mammals distinguished by a peculiar pouch 

 or duplicature of the abdominal integument, which in the males is 

 everted, forming a pendulous bag containing the testes ; and in the 

 females is inverted, forming a hidden pouch containing the nipples 

 and usually sheltering the young for a certain period after their 

 birth: they have the marsupial bones in common with the Mo- 

 notremes; a much-varied dentition, especially as regards the 

 number of incisors, but usually including 4 true molars ; and never 

 more than 3 premolars* : the angle of the lower jaw is more or 

 less inverted f. 



With the exception of one genus, Bidelphys, which is American, 

 and another genus Cuscus, which is Malayan, all the known exist- 

 ing Marsupials belong to Australia, Tasmania, and New Guinea. 

 The grazing and browsing Kangaroos are rarely seen abroad in full 

 daylight, save in dark rainy weather. Most of the Marsupialia are 

 nocturnal. Zoological wanderers in Australia, viewing its plains 

 and scanning its scrubs by broad daylight, are struck by the seem- 

 ing absence of mammalian life ; but during the brief twilight and 

 dawn, or by the light of the moon, numerous forms are seen to 



* " Outlines of a Classification of the Marsupialia," Trans. Zool. Soc. vol. ii.. 

 1839. 



t For other Osteologieal and Dental characteristics of the Marsupialia, see 

 the paper above cited, and that " On the Osteology of the MarsupiaUa," Trans. 

 Zool. Soc. vol. ii. p. 379 (1838). 



