64 



COMPARATIVE ANATOMY 



ossified ; while the lateral portions remain cartilaginous. In 

 other Marsupials various stages in the reduction of the episternum 

 are met with. 



Amongst the Placeiitalia 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 Akch of Ornithorhynchus paradoxus. 



m.s, manubrium slerni ; c', c^, c^, first, second, and third ribs ; nt, sternebra ; sc, 

 scapula ; m.c, coraooid (Inetacoracoid) ; e.c, epicoracoid ; cl, clavicle ; es^ nd 

 en'-, episternum {" interolavicle"). 



cartilaginous portion is closely applied to the sternum, while the 

 lateral portions are connected with the clavicles. 



In the Soiuromorphse and Myomorphae 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 SciuromorphEe articulate with the clavicles. 

 In the Lagomorphse fibro-oartilaginous lateral portions only are present, 

 extending as far as the clavicles. 



Y. THE SKULL. 



Iiito-odicction. 



The question as to the primary origin of the skull ia 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 



