Chap. III. GENERA OF CnELYDROID.E. 413 



greatest force to the bite of the animal : the mouth is narrow ; tlie jaws are 

 strong ; and tlieir muscles are enormously developed, forming the great bulk of the 

 head. In Chelydra the mouth is broader, the jaws are not so strong, and their 

 muscles are less developed. Upon this general dillerence depend most of the dis- 

 tinguishing characters of the two groups. 



I. Gypochelys, Ag. 



The skull of Gypochelys is very broad and high at the hind end, and rap- 

 idl}- grows narrow and low thence forward ; that part which includes the mouth 

 and eyes and nose being very small in comparison with that which includes the 

 fossjB temporales. The upper surface is nearly horizontal from side to side, and 

 meets the sides at sharp angles ; it descends steeply from behind forward till 

 between the eyes, where it makes an angle, and thence to the front end it is 

 nearly horizontal ; it narrows continually forward from where it first reaches 

 entirely across the head, but is still broad between the eyes, and blunt at the 

 front end. The sides spread outward somewhat towards the lower edge between 

 the ears and eyes, (that is, over the fossae temporales,) and thus the head grows 

 broader downward ; but, in front of the fosste, the head is broader across the 

 upper surface than across the mouth below. The eyes open sidewise and forward, 

 not at all upward ; the sides of the nasal region in front of the eye are nearly 

 vertical ; and the outer surface of the jaw is turned inward toward the alveolar 

 edge, except at the symphysis, where it is on a nearly vertical line with the 

 end of the nose above. Thus the mouth is narrow. The nasal region is high, 

 and flattened sidewise. The upper jaw, at the symphysis, is drawn down to a long, 

 strong point. On each side of this point the alveolar edge rises steeply, then 

 curves down under the eye, and again a little upward at the hind end. The 

 alveolar surface is carried high up under the nose, so as to form there an inverted, 

 deep, conical pit. The pterygoids are narrow between the muscles of the jaw. 

 The lower jaw is high and strong; and, like the upper one, it is drawn out at 

 the symphisis to a long, strong point, which rises higher than the coronal angle. 

 The outer surface, at tlie S3'mphysis, curves far iiuvanl in descending from the 

 upper to the lower edge, and, when the mouth is closed and the point of this 

 jaw carried to the top of the pit above, there is a large space in front of this 

 surface between it and the inner surface of the upper jaw. The strength of the 

 jaw-s the height of the lower one, the height of the head over the mouth, the 

 narrowness of the mouth itself, and the height and width of the back part of the 

 head, are all clearly connected with the force of the bite of the animal. The 



