622 CHORDATA. 



two sides are pressed together to a single bone lying vertically 

 entirely within the nasal partition; the palatine and pterygoid are 

 forced backwards. The palatines contribute to the hard palate, 

 the pterygoids only exceptionally (Cetacea, many edentates); the 

 latter usually lose their independence and fuse with the nearest 

 bone of the base of the cranium, the basisphenoid (more accurately 

 with a process of the basisphenoid, the lamina externa of the 

 pterygoid process, the pterygoid forming the lamina interna). 

 Thus the hinder sphenoid, like the temporal, contains cranial and 

 visceral elements. 



In the vertebral column the cervical and the rib-bearing 

 thoracic vertebrae are always distinct, and the same, with the ex- 

 ception of the Cetacea and Sirenia, is true of lumbar, sacral, and 

 caudal vertebrae. Of sacrals there is one in all embryos, and 

 throughout life in the marsupials, elsewhere from two to five, 

 rarely, as in edentates, as many as thirteen. The number of ver- 

 tebrae in each group is rather restricted. Thus, except in Brady- 

 pus tridactylus (9), Cholcepus hoffmanni and Manatus (6), the 

 number of cervicals is always seven. 



Of the appendicular skeleton the girdles are most interesting. 



The coracoid, which in mono- 

 tremes reaches the sternum, is 

 reduced in all other mammals to 

 a small coracoid process of the 

 scapula. More rarely the clavicle 

 is lacking (rapid runners); in 

 the monotremes it extends to the 

 episternum (fig. 648, Cl, Ep)\ 

 elsewhere it appears to articulate 

 with the sternum, in reality by 

 the intervention of interarticular 

 cartilages (once regarded as a 



FIG. 648. Sternum and shoulder girdle 



of Ornithorhynchus paradoxm. (From rudimentary episternum, now 



Wiedersheim.) Cl, clavicle; Co, Co', i N T 



coracoid; Ep, episternum; G, glenoid Called preclaviae). In the pelvis 



fossa for humerus; S, scapula; St, ,, ,-, , , * . 



manubrium sterni (anterior element all three elements are lUSed to a 



of sternum). i -i 



single os mnommatum; pubis and 



ischium unite ventrally with each other, enclosing between them 

 the obturator foramen (fig. 655). The pubes of the two sides 

 unite by a symphysis which can extend back to the ischia. 



Since the mammals in general are distinguished from other 

 vertebrates by their intelligence, the brain is characterized by the 

 size of cerebrum and cerebellum (fig. 649). In contrast to birds 



