20 



THE MAMMALIA 



[SECT. A 



III. Each of the long bones of the limbs (in the young con- 

 dition) is composed of a central part or shaft, and of terminal 

 epiphyses, the latter only becoming completely united with the 

 shaft at an advanced stage. 



IV. The ilio-sacral connection is pre-acetabular : there is a 

 cruro-tarsal joint. 



Squamosal 



Jugal(malar) 

 Quadrate 

 Mandible 



Jug a I (malar) 



-Squamosal 

 Mandible 



Single median condyle 

 Fie. 6. 



Occipital condyles 



Fig. 7. 



Fig. 6. Occipital view of the cranium of a Bird (Larus) ; to shew the single 

 median occipital condyle, and the intervention of the quadrate bone between the 

 mandible and the cranial wall. 



Fig. 7. Occipital view of the cranium of a Dog ; to shew the twin occipital con- 

 dyles, and the direct articulation of the mandible with the cranial wall, witbout the 

 intervention of a quadrate bone. 



V. In the Mammalian pectoral girdle, the coracoid of Birds 

 and Reptiles is usually represented only by a vestige or vestiges 

 which unite with the scapula in the adult. [Figs. 8, 9, and 10.] 



VI. Mammals are typically diphyodont, i.e. have two sets of 

 teeth— a milk or deciduous set, and a permanent set : some are 

 monophyodont, i.e. have only one set. The teeth are thecodont, 

 i.e. the base of each tooth is embedded in a distinct socket or 

 alveolus in the substance of the bone of the jaw : and nearly 

 always the teeth in different parts of the jaw are clearly dis- 

 tinguishable by differences of shape into incisors, canines, and 

 grinding teeth, i.e. are heterodont ; in some instances the teeth 

 are all alike (homodont). 



VII. A cloaca is absent, except in the Prototheria. 



