530 MARSUPIALIA. 



septum and, except in Perameles, of an allantoic placenta. 

 As a rule only one tooth (the last premolar) is replaced ; the glans 

 penis is generally bifurcated, and the crura penis are not united 

 to the ischia. 



The odontoid process fuses early with the axis, and the cervical ribs 

 with their respective vertebrae. There are usually nineteen dorso-lumbar 

 vertebrae of which the anterior thirteen bear ribs. The sacrum consists 

 of two vertebrae, but it may be reinforced by caudal vertebrae. The 

 palate has large posterior vacuities. The alisphenoid assists in forming 

 the tympanic bulla. The carotid canal perforates the basisphenoid. The 

 tympanic, periotic and squamosal remain separate. The jugal reaches 

 back to the glenoid cavity and participates in forming it. The angle 

 of the lower jaw is usually inflected (except in Tarsipes). The teeth vary 

 in structure according to the mode of life. The usual formula is i ~ 

 G \ p f m f . As may be seen from the formula the number of incisors 

 and molars is larger than in other mammals. The grinders are either 

 secodont, bunodont or lophodont, and canines are often absent in the 

 herbivorous forms. In some mesozoic forms the canines are two-rooted. 



The marsupials are peculiar in the fact that only one tooth the last 

 premolar is replaced. In some forms this tooth remains until the 

 animal is full-grown (e.g. Potorous) ; in others (e.g. Thylacinus) it is 

 reduced and is absorbed or shed before the other teeth appear. In 

 others again it has not been detected at all, and there is no replacement 

 {Wombat, Dasyurus, Myrmecobius). In the extinct Sparassodontidae two 

 premolars and the canines are replaced. Various answers have been 

 given to the question whether the teeth belong to the permanent or 

 milk dentition (see Table on p. 508). By some it is held that all 

 the teeth belong to the milk series, except the tooth which replaces 

 the last premolar, which is regarded as belonging to the permanent series.* 

 According to this the permanent series must be supposed to have become 

 abortive, a view which is supported by the fact that tooth germs not 

 attaining full development are formed in the position of the permanent 

 tooth germs of the completely diphyodont mammals. However this 

 may be, there is undoubtedly a set of tooth germs which are developed 

 before those of the persistent teeth. These have been called by the up- 

 holders of the above-mentioned view prelacteal. Others hold that the 

 deciduous tooth and the persisting teeth belong to the permanent den- 

 tition, and that the deciduous tooth is the first of the molar series which in 

 consequence of the shortening of the jaw has been overlapped by the 

 fourth premolar which pushes it out (Tims). 



The coracoid is reduced to a process of the scapula, as in the higher 

 forms. A clavicle is present except in the Peramelidae. There is no 

 interclavicle. The bones of the forearm are separate and generally 

 adapted for pronation and supination. The thumb is not opposable, 

 but the two inner digits of the manus can frequently be opposed to the 

 three outer in grasping. The manus possesses five digits except in Choe- 

 ropus, and the carpus is without an os centrale Epipubic bones (Fig. 

 276,'M) project forwards from the pubis in all except Thylacinus in which 



* By some even this is regarded as belonging'to the milk series (between 

 the last and penultimate premolar), and arising late (M. Woodward, 

 P.Z.S., 1893, p.J4-67). 



