160 THE VERTEBRATE SKELETON. 



part of which is formed of bone, the apical part of dentine, 

 capped by a very weak development of enamel. 



The posterior end of the maxilla is overlapped by a small 

 bone, the quadratojugal (figs. 24 and 26, 9), whose posterior 

 end forms part of the articular surface for the lower jaw. 

 Just behind the quadratojugal there is a small unossified area 

 which lies at the angle of the mouth, and is connected by a 

 narrow bar of cartilage with the cranium ; this forms the 

 quadrate (figs. 24 and 26, 19). A backwardly-directed out- 

 growth from the cartilaginous bar more or less completely 

 surrounds the tympanic membrane, forming the tympanic 

 ring. When followed back the maxilla and quadratojugal 

 diverge further and further from the cranium, till the angle 

 of the mouth comes to be separated from the foramen magnum 

 by a space nearly double the width of the cranium. This space 

 is bridged over to a considerable extent by two triradiate bones, 

 the pterygoid and squamosal. 



The pterygoid (figs. 24 and 26, 3) is a large bone, whose 

 anterior limb runs forwards meeting the maxilla and palatine; 

 while its inner limb meets the auditory capsule and para- 

 sphenoid, and its outer limb runs backwards and outwards 

 to the angle of the mouth. The palatine is a small transversely- 

 placed bone, which connects the pterygoid with the anterior 

 part of the sphenethmoid. The squamosal (figs. 24 and 26, 4) 

 is a T-shaped bone whose anterior arm is pointed and passes 

 forwards to meet the pterygoid. The posterior upper arm is 

 closely applied to the pro-otic, while the posterior lower arm 

 meets the pterygoid and quadratojugal at the angle of the jaw, 

 and surrounds the narrow cartilaginous bar of the quadrate 

 which goes to join the cranium. The squamosal is probably 

 homologous with the squamosal together with the pre-opercular 

 of Bony Ganoids. 



The quadrate and squamosal form the suspensorium. by 

 which the lower jaw is connected with the cranium. 



The lower jaw or mandible consists of a pair of carti- 

 laginous rods (Meckel's cartilages) in connection with each 



