THE FROG 29 



is formed by the two jaws, both of which are first laid down in carti- 

 lage. The upper jaw in the adult, however, is covered by three 

 membrane bones. The quadrato-jugal is a short bone sheathing the 

 jaw at its hinder end and bearing no teeth. ! From this the much 

 longer maxilla runs forward, and lastly, the front part is completed 

 by a smaller bone, the pre-maxilla, which joins its fellow in the middle 

 line and also sends a small dorsal process backwards to form the 

 inner boundary of the external nostril. Both the pre-maxilla and 

 maxilla bear a row of small sharp conical teeth. The upper jaw 

 is immovably fixed to the cranium at the posterior end and again 

 near the anterior end. The hinder connection is made by the 

 quadrate cartilage, or, as it is also termed, the suspensorium. This 

 is a cartilaginous rod, forked at its inner end, that runs outwards 

 from the side of the auditory capsule and bears at its outer end a 

 hollow, glenoid cavity, with which the lower jaw articulates. The 

 dorsal side of the suspensorium is covered by one limb of a large 

 tri-radiate membrane bone, the squamosal, of whose other limbs one 

 runs to the pro-otic bone and the other forwards towards the second 

 point of attachment of the upper jaw. A similar triradiate bone, 

 the pterygoid, in part underlies the quadrate cartilage. Its three 

 limbs are similarly distributed, and the anterior one reaches and 

 joins with the upper jaw. The anterior attachment of the jaw is also 

 formed by a bar of cartilage, the palatine cartilage, passing outwards 

 from the sphenethmoid bone. This is underneath the hinder edge of 

 the nasal bone and encased on the ventral side by a slender mem- 

 brane bone, the palatine bone. Like the pterygoid, the palatine bone 

 in the higher animals is a cartilage bone, but this is not so in Rana. 

 The lower jaw or mandible is similarly a cartilaginous arch, 

 Meckel's cartilage, almost entirely covered by membrane bones, 

 none of which bears teeth. The posterior covering bone is the short 

 angulo-splenial, which possesses a small articular knob, and in front 

 of it is situated the long dentary. At the anterior end in young 

 frogs a small piece of cartilage is left, but in older skulls this ossifies 

 to form a small bone, the mento-meckelian, and the two halves of the 

 jaw are bound together by a tough ligament. 



The hyoid cartilage, or body of the hyoid, situated in the 

 floor of the mouth, is a thin shield-shaped plate, giving off processes 

 at its fore and hind ends. The anterior processes, or cornua, are 

 very slight and cartilaginous, and pass first forwards, then backwards 

 and upwards. They are finally attached to the back of the auditory 

 capsules, and the columellae are formed from small pieces separated 

 off from their anterior ends. The posterior cornua are short some- 

 what stout rods of cartilage bone, that pass back one on each side of 

 the glottis. 



