64 COMPARATIVE ANATOMY 



ossified; while the lateral portions remain cartilaginous. In 

 other Marsupials various stages in the reduction of the episteraum 

 are met with. 



Amongst the Placentalia the episternum is retained in the most 

 independent condition in certain South American Cavies as well as 

 in the Porcupine and other Rodents in which it consists of a median 

 and two lateral parts, which are, however, quite independent of one 

 another, and are only connected by ligaments. The median, 



FIG. 48. PECTORAL ARCH or Ornithorhynchus paradoxus. 



m.s, manubrium sterni ; c 1 , c 2 , c 3 , first, second, and third ribs ; at, sternebra ; sc, 

 scapula ; ra.c, coracoid (metacoracoid) ; e.c, epicoracoid ; d, clavicle; es 1 nd 

 es 2 , episternum (" interclavicle"). 



cartilaginous portion is closely applied to the sternum, while the 

 lateral portions are connected with the clavicles, 



In the Sciuromorphse and Myomorphse the episternal apparatus is still 

 further modified, the median piece having disappeared (or more probably 

 having united with the sternum), while the small lateral pieces are attached 

 to the manubrium and in the Sciuromorphre articulate with the clavicles. 

 In the Lagomorphse fibre-cartilaginous lateral portions only are present, 

 extending as far as the clavicles. 



V. THE SKULL. 



Introduction. 



The question as to the primary origin of the skull in the 

 Craniata has always taken a foremost place amongst the morpho- 

 logical problems relating to the structure of Vertebrates. Until 

 past the middle of the present century the theory which held the 

 field was the "vertebral theory" of Goethe and Oken, according to 



