THE SKULL 297 



bone " of Ornithorhynchus. The mammalian vomer repre- 

 sents the anterior part of the parasphenoid of lower forms. 



In the lower jaw, Meckel's cartilage ossifies as the articular, 

 and dermal bones are formed round it. In the lower verte- 

 brates these dermal bones are numerous, consisting in the 

 Stegocephalia, for instance, of the dentary, angular, supra- 

 angular, splenial, and three coronoid bones. The number of 

 these bones becomes reduced in higher forms. The two 

 halves of the lower jaw in snakes are separate, and their front 

 ends can be moved wide apart. This allows the mouth to be 

 opened very wide indeed, so that the snake is capable of 

 swallowing relatively enormous prey. In some Lacertilia such 

 as Varanus and in the extinct Mosasauria, there is a joint on 

 each side of the lower jaw. These joints enable the space 

 between the two halves of the lower jaw to be widened, and 

 large prey to be swallowed. 



In crocodiles, and in the fossil bird Archaeopteryx, the 

 lower jaw is characterised by being pierced by a foramen on 

 each side. Among the Dinosauria, the Predentata are peculiar 

 in possessing a predentary bone, the most anterior in the lower 

 jaw. The lower jaw of the Marsupials is characterised by the 

 fact that the lower edge of the hindmost region of each half is 

 bent inwards, forming the " inflected angles." 



In all mammals the lower jaw is peculiar in consisting of a 

 single bone : the dentary. Very interesting stages in the 

 reduction in number of bones are found in the Theromorph 

 reptiles. Cynognathus has a large dentary, while the articular, 

 angular, supra- angular, prearticular, coronoid, and splenial are 

 small. The dentary develops an uprising coronoid process 

 which touches the squamosal, and so takes on the function of 

 articulating the lower on to the upper jaw. At the same time 

 the original quadrate-articular articulation (which is present 

 in all lower forms) falls into disuse, and the quadrate becomes 

 small and loose. The next stage is that of the mammals, of 

 which the dog may be taken as an example ; and since here the 

 lower jaw consists of the dentary alone, the question arises as 

 to what has happened to the other bones. The quadrate and 

 articular have been intercalated between the columella auris 



