510 



MR. N. A. MACKINTOSH ON THE CHONDROCHANTUM 



this respect it resembles the lower jaw of Amia. In place of the 

 temporal cartilage we have now the ' clearly differentiated 

 hyomandibular, symplecbic, and quadrate. The hyomandibular 

 presents a rather unusual appearance. It is club-shaped and 

 very long. At its point of articulation with the auditory capsule 

 it has a broad plate-like head which is continued downwards as a 

 comparatively slender stem mei-ging into the symplectic. The 

 latter is also rather longer than usual and runs down close to 

 the lower margin of the quadrate. This is a very large plate of 

 cartilage extended upward and backward as a metapterygoid 

 process. Posteriorly the quadrate is in intimate association with 



Text-figure 8. 



T.T.WI 



Aud c. 



Stage 4. Dorsal view of chondrocranium. 



the sj'^mpleotic but is not continuous with it. The upper jaw, 

 consisting of the palato-pterygoid cartilage, is now esl-ablished and 

 runs from the upper anterior angle of the quadrate up to a point 

 level with the anterior end of the rostral cartilage. The posterior 

 end of the palato-pterygoid is very slender. It passes close to the 

 mai'gin of the ethmoid plate but is not articulated with it. At 

 its extremity it turns outwards (text-fig. 8), reaching almost to 

 the surface of the snout. 



The fact that the upper jaw is connected with the quadrate 

 only by a very slender piece of cartilage suggests that it may have 

 arisen independently and grown back to meet the dorso -anterior 



