344. AMERICAN TESTUDINATA. Part II. 



The head is large ; it is narrow about the nose and eyes, but grows rapidly- 

 broad backward to the ear region. The floor of the skull, that is, the roof of 

 the mouth and the base of the brain-box, taken as a whole, is on nearly a hori- 

 zontal plane ; the top of the skull in passing forward over the brain descends 

 as steeply, and in Gypochelys Temminckii much more steeply, than in passing over 

 the front part of the head, so that we have here none of tlie angle which in 

 the Trionychidaj is caused by the turning down of the front part of the skull. 

 The ear region is broad from the brain-box outward, but short from behind for- 

 ward. The mastoid is short ; its hind surface reaches more upward than back- 

 ward, and the os quadratum below descends in nearly a line with it ; thus the back 

 of the head is high, broad, and square. The crest on the brain-box is high. 

 The pterygoids are narrow, and their edges are deeply concave. The breadth of 

 the ear region, the height of the crest, and the narrowness of the pterygoids, unite 

 to give room for the attachment and passage of very large temporal muscles. 

 The arch from the ear to the eye, made up of the jugal, postfrontal, and tem- 

 poral bones is broad ; the parietals project sidewise, and, for some distance back of 

 the eyes, unite with the postfrontals in making a continuous arch over the head ; 

 moreover the ojDcnings for the eyes and nose are small. Thus the head is much 

 more protected by bone than in any other family of the sub-order, but much 

 less than in the sea Turtles, for there the bony arch reaches to the hind 

 extremity of the head, whereas here the ear region is exposed from above. The 

 sphenoid is short, and does not extend nearly the whole length of the pterygoids. 

 The jaws are strong ; they have sharp alveolar edges, and are pointed at the 

 symphyses. 



The free skin is loose, and very movable on the neck and limbs ; it does 

 not close around the legs above the knees and elbows, and below incloses them 

 only loosely. The shield is covered with large horny epidermal scales, the arrange- 

 ment of which presents rather generic than family characters, especially those of 

 the plastron. The free skin, where it is most exposed, especially on the under 

 surfaces of the limbs, on the whole front limbs below the elbows, on the neck 

 just behind the head, and on the tail, thickens at numerous points into a kind 

 of tubercles, and on these tubercles the epidermis is hardened into a kind of scales. 



of the body takes in locomotion, in this family, re- Testudinata during tlieir earlier stages of develop- 



minds us rather of the character of the whole ver- ment. This resemblance of the Chelydroids and 



tebral column in the other Reptiles, in which it con- other Reptiles is no doubt hinted at in the vernac- 



stitutes the principal organ of locomotion. Thus we ular name under which the most common North 



have here a character which is rather Reptilian than American species is known all over the southern 



Chelonian ; and this coincides remarkably with the United States, where it is called Alligator-C'outa, 



comparatively greater length of the tail in all the from the similarity of its tail to that of an Alligator. 



