92 MORPHOLOGICAL TYPES 



first two branchial arches form a framework often called the 

 " hyoid," lying under the floor of the mouth, and of importance 

 in breathing. When this floor is lowered, air is drawn in 

 through the nostrils. These are then closed and the floor 

 raised, with the result that the air has to pass down the throat 

 and into the lungs. 



Teeth are carried by the premaxilla, maxilla, prevomer, 

 and dentary. They are similar to the teeth of fish already 

 described, but now they are firmly fixed on to the bones which 

 carry them. 



The vagus nerves emerge just in front of the back of the 

 skull, and the hypoglossal roots emerge behind it. Six 

 segments are included in the skull. 



Vertebral Column . — The vertebrae are elongated cylinders, 

 articulating with one another by zygapophyses and cartilaginous 

 pads on their front and rear surfaces. The notochord is much 

 constricted. Each vertebra bears a pair of neural arches 

 above, and those of the tail also have haemal arches beneath. 

 The trunk- vertebrae (except the first) bear lateral transverse 

 processes, with which the ribs articulate. The latter are 

 " true " or dorsal ribs. The first vertebra is modified in 

 connexion with its articulation with the condyles of the skull. 

 The ribs belonging to one vertebra are modified and attached 

 to the ilia of the pelvic girdle, forming the sacrum. 



Pectoral Limb and Girdle. — The pectoral girdle is 

 represented only by the primitive cartilaginous girdle of the 

 endoskeleton, and indeed it remains largely unossified. The 

 dermal or membrane-bones of the girdle of other forms have 

 disappeared. There is a dorsal scapula, and a ventral portion 

 in which these may be distinguished : an anterior cartilaginous 

 precoracoid and a posterior partly ossified coracoid. A sternum 

 is present as a median plate of cartilage, overlapping the 

 coracoid. 



The fore limb is divisible into three regions : upper arm, 

 forearm, and hand. The skeleton of the limbs is derived from 

 the radials of the fins of fish, and consequently its bones are 

 cartilage-bones. The bone in the upper arm is the humerus, 

 which fits into the glenoid cavity of the girdle, proximally. 



