ORDER MONOTREMATA. 



1 27 1 



of epipubic bones 1 (fig. 1139, a«), but differ in the more specialised 

 characters of the auditory ossicles ; in the reduction of the coracoid 

 to a mere process of the scapula, and its 

 non-articulation with the sternum ; in the 

 absence of an interclavicle ; and in the pres- 

 ence of nipples to the mammary glands. 

 In the latter characters they agree with the 

 Eutheria, from which they are distinguished 

 by the presence of an imperfect cloaca, 

 and by the characters of the urinogenital 

 organs, which are in some respects inter- 

 mediate between those of the Eutheria 

 and Prototheria. The young, although 

 produced viviparously, are born in an ex- 

 ceedingly imperfect condition, and are 

 never nourished in utero by an allantoic 

 placenta ; but at an early period are trans- 

 ferred to the nipples of the mother, to 

 which they adhere for a long time. The 

 nipples themselves are nearly always con- 

 cealed in a fold of skin forming the so- 

 called marsupium. 



Teeth are invariably present, and are 

 divisible into the usual four groups. With the exception of the 

 Phascolomyidtz, the number of incisors in the upper jaw of all exist- 

 ing forms exceeds that in the mandible. 2 The true molars are very 

 generally four in number on either side of each jaw ; and in all 

 existing forms the number of premolars does not exceed three, 

 although four (which may be taken as the typical number) are 

 found in several Mesozoic genera. The most remarkable feature 

 about the dentition is, however, that it is only the last premolar (the 

 fourth of the typical series) among the whole number of teeth that 

 ever has a milk predecessor ; such predecessor generally resembling 

 the true molars in structure. Some genera, like Phascolomys, show, 

 however, no signs of even this single replacement ; and it is pretty 

 clear that we have here the first commencement of a replacing series 

 of teeth, which in the early Eutheria must have gradually extended 

 anteriorly, until it normally embraced all the teeth in advance of 

 Pm. 4, with the very general exception of Pm. 1 , although in certain 

 Perissodactyla even that tooth had a milk predecessor. 



Another very characteristic, although not universal, feature in this 

 subclass is the inflection of the angle of the mandible (fig. 1152) ; 



1 Unossified in Thylacinus. 



2 The same feature occurs among the Anomodont Reptiles in the Galesaurida. 



Fig. 1139. — Ventral view of the 

 symphysis and left half of the 

 pelvis of a Kangaroo, m, Epi- 

 pubic bones. Reduced. 



VOL. II. 



2 B 



