SKELETON. 637 



quadrate, lower jaw, columella, and hyoid. The surface is 

 polished ; the sutures are obliterated very early in life. 



The back part of the skull is formed by the basioccipital, 

 the two exoccipitals, and the supraoccipital, surrounding 

 the foramen magnum. The basioccipital forms most of the 

 single condyle. 



The roof of the skull is formed from the paired parietals, 

 frontals, and nasals, the last being small and in part super- 

 seded by the upward extension of the premaxillae. 



The line of the upper jaw consists of premaxilla, small 

 maxilla, jugal, and quadrato-jugal, the last abutting on the 

 movable quadrate. 



Of the membrane bones on the side of the skull, the 

 lachrymal in front of the orbit, and the squamosal between 

 the quadrate and the parietal, are the most important. 



On the roof of the mouth, the basisphenoid, which lies 

 just in front of the basioccipital, is covered over by a 

 membrane bone the basitemporal. In front of this is a 

 sharp " basisphenoid rostrum " or parasphenoid, also a mem- 

 brane bone. Articulating with the quadrate and with the 

 rostrum are the pterygoids, in front of these lie the palatines. 

 The vomer is vestigial. The bony front of the palate is 

 formed from inward extensions of the premaxillee and 

 maxillce. The interorbital septum is formed chiefly from 

 the mesethmoid, but also from the presphenoid. From the 

 tympanum to the inner ear extends the rod-like columella. 

 The lower jaw originally consists of four membrane bones 

 -dentary, splenial, angular, and surangular ; and one carti- 

 lage bone the articular. The hyoid consists of a flat 

 "body," with anterior and posterior "horns," the latter 

 derived from the first branchial arch. 



The pectoral girdle consists of sabre-like scapulae extend- 

 ing dorsally over the ribs, of stout coracoids sloping ventrally 

 and articulating with the sternum, of the clavicles which are 

 united by the interclavicle to form the merrythought or fur- 

 cula. The opening left where the upper ends of the clavicles 

 touch the scapula and coracoid is called faeforamen triossciun. 



The sternum bears a conspicuous keel, is produced 

 laterally and posteriorly into two xiphoid processes, and 

 bears articular surfaces for the coracoids anteriorly, for the 

 sternal ribs laterally. 



