SKULL MAMMALS 199 



duct, its perpendicular plate lying behind the vertical plate of the 

 palatine and extending to the pterygoid process of the ahsphenoid 

 behind. Its inferior end, the hamulus, is preformed in cartilage 

 and becomes intimately united with the rest. The pterygoid lacks 

 a palatine process in most mammals, but whales have a strong one, 

 and in Myrmecophaga those of the two sides meet in the middle line, 

 carrying the choanas back nearly to the foramen magnum. 



The pterygoid bone is the internal pterygoid process of man. It persists 

 separately in most mammals, although closely connected externally with the 

 pterygoid process of the alisphenoid, often with an ectopterygoid groove between 

 them, while a mesopterygoid fossa lies between the .right and left pterygoids. 

 It is possible that the isolated hamular cartilage or the median lamella of the 

 alisphenoid is the homologue of the pterygoid process of the pterygoquadrae 

 of lower Vertebrates. These parts are widely separated from the quadrate 

 in mammals. 



The pterygoid enters the cranial wall in Monotremes, but not elsewhere. 

 This part of the pterygoid, according to Gaupp, is the only homologue of the 

 Saurian pterygoid, the rest of the bone being compared to the posterolateral 

 parts of the parasphenoid. Further study is needed. 



The quadrate is considered below in connexion with the ossicula 

 auditus (p. 201). The rest of the mandibular arch (Meckel's carti- 

 lage) articulates behind with the incus, the two halves meeting in 

 front. Two bones ossify in either half of this cartilage lower jaw, a 

 mentomeckehan at the anterior end (not known in all mammals) 

 and a malleus behind in the articular region, the latter separating 

 from the rest of the Meckelian and becoming one of the bones of the 

 middle ear. 



In the adult there is a single bone in either half of the lower jaw, 

 usually called the dentale, the mentomeckelian being absorbed in 

 its front end. The right and left halves meet in front and may be 

 connected by hgament or cartilage, or they may ankylose at the 

 symphysis (Perissodactyls, elephants, bats, toothed whales, apes and 

 man. This dentale terminates behind in a condyle^ which articulates 

 with the mandibular fossa of the squamosal. In some lowermammals 

 and in those where the muscles are weak, the condylar part is nearly 

 in line with the rest of the bone; elsewhere it is bent at an angle with 

 the rest, forming the ramus of the jaw, the upper posterior part of 



1 In forms with a mixed diet the condyle is more or less globular, allowing a certain 

 amount of rotation of the lower jaw. Some (especially Carnivores), have it trans- 

 versely cylindrical (roller), permitting only an up and down motion, while, more rarely 

 it is elongate in the plane of the jaw, allowing only a fore and aft motion. 



