234 THE DOG-FISH. 



1. The first or mandibular arch is very greatly modified; 



its ventral part is large and stout and forms the 

 lower jaw. The upper jaw is formed as an out- 

 growth from its anterior edge. 



a. The upper jaw consists of a pair of stout carti- 



laginous rods, wide behind and narrowing 

 towards their anterior ends, where they are 

 united across the middle line by ligament. They 

 extend forwards as far as the openings of the 

 olfactory capsules, and are connected by stout 

 ethmo-palatine ligaments with the base of the 

 skull, at the anteroinferior angles of the orbits. 

 They bear teeth along the ventral surface of the 

 anterior two-thirds of their length, but these are 

 lost in skulls prepared as described. 



b. The lower jaw consists of a pair of wide flattened 



bars of cartilage, which are movably hinged with 

 the upper jaw behind, and bound together by 

 a median ligament in front. It lies, when the 

 mouth is shut, parallel to and immediately 

 behind the upper jaw. It bears teeth along the 

 dorsal surface of the anterior two-thirds of its 

 length on each side, but these are lost in skulls 

 prepared as described. 



2. The second or hyoidean arch is incomplete above : it 



consists of a pair of rods the dorsal ends of which 

 articulate with the sides of the skull, while the 

 ventral ends are connected together by a median 

 plate of cartilage. Each rod is further divided into 

 an upper and a lower segment. 



a. The hyo-mandibular cartilage, or upper segment, is 

 a short stout bar of cartilage, projecting almost 

 horizontally outwards from the skull, but slightly 

 downwards and backwards. Its inner and upper 

 end articulates with a concave surface on the side 

 of the skull, near its hinder end, and immediately 



