18 GUIDES FOR VERTEBRATE DISSECTION 



is formed by a pair of premaxillary bones which meet at an 

 angle, each sending backwards an ascending process which 

 partly covers the anterior end of the frontal of the same side. 

 The rest of the premaxillary is formed by a tooth-bearing alveolar 

 process. 



Wedged into the angle between the antero-lateral part of the 

 frontal and the alveolar process of the premaxillary is the upper 

 surface of the vomer, the posterior portion of which is covered 

 by the cartilaginous antorbital process of the chondrocranium 

 to be studied later. 



Extending obliquely outwards and forwards from the external 

 surface of the opisthotic is a slender splint-like squamosal bone 

 which articulates in front with the quadrate, while at about the 

 middle of its lower margin it gives off a short opercular process 

 which articulates with the stapes (infra). The quadrate, just 

 referred to, is a splint-like bone just inside the squamosal and 

 expanded at its anterior end where it bears a shallow fossa with 

 which the lower jaw is articulated. The posterior portion lies 

 close to the quadrate cartilage from which it has arisen by ossi- 

 fication. 



At its posterior end the quadrate meets the lateral surface of 

 the prootic bone, parts of which are visible in both dorsal and 

 ventral views of the skull, the more medial portions being covered 

 by the parietals above and the parasphenoid below. 



Extending inwards and forwards from the quadrate to the 

 vomer is the palatopterygoid bone, the limits of which are best 

 seen in the vqntral view of the cranium. 



Draw a dorsal view of the skull X4. 



On the ventral surface of the cranium in the middle line is a 

 large parasphenoid bone, the anterior end of which lies between 

 the palatine processes of the two vomers. The vomers (already 

 seen from above) present, besides this palatine process, a tooth- 

 bearing alveolar process which articulates behind with the 

 anterior end of the palatopterygoid, this latter having teeth in 

 front continuous with those of the vomer. The palatopterygoid 

 articulates behind with the quadrate bone and the quadrate 

 cartilage. 



In drawing the ventral view of the cranium, insert these 

 parts as well as the quadrate, prootic, opisthotic, and exoccipital 

 bones recognized in the dorsal view. 



