408 CHELONIA 



CHAP. 



worn down ; and broad, thick, horny, crushing pads are developed 

 in their stead, the supporting parts of the jaws becoming more 

 massive. The masticatory muscles are likewise enlarged, and a 

 tubercle grows upon the lower border of the jugal bone, whence 

 arises part of the masseter muscle. 



The eggs are round, thick -shelled, but very brittle ; they are 

 laid in the sand above the level of the water, and this is the 

 chief occasion on which these tortoises creep on land. 



Trionyx. The plastron has no special cutaneous valves for 

 the concealment of the hind-limbs. This is the principal genus, 

 with the greatest number of species and the widest distribution, 

 the latter coinciding with that of the whole family. The upper 

 surface of the shell of young specimens frequently forms numerous 

 longitudinal ridges or series of little horny tubercles which dis- 

 appear with age. 



T. ferox, the commonest " Soft-shelled Turtle " of the United 

 States. Olive above with scattered, small, round, black spots ; 

 young with conical, spine-like tubercles, especially on the nuchal 

 border and on the posterior portion of the shell, which has a 

 pale, black-edged border. A light, black-edged streak passes 

 through the eye and joins its fellow on the snout. The limbs 

 are olive brown, spotted and marbled with black. The under 

 parts of the shell are white. Very large specimens have a shell 

 18 inches in length and 16 inches wide. Holbrook gives the 

 following account of its habits : 



" A voracious, carnivorous creature. They reside most con-, 

 stantly in the water, swim with rapidity, and choose for their 

 retreat holes under the banks of rivers, or under rocks ; and 

 not unfrequently the trunk of some huge forest tree, fallen 

 into the stream, affords them shelter. Sometimes they leave the 

 water and conceal themselves in the mud: I have frequently 

 seen them thus buried to the depth of 2 or 3 inches, leaving 

 only a small breathing hole for the long neck and narrow head, 

 which is occasionally thrust out, but most commonly it is 

 retracted so that one would pass near without observing their 

 habitation ; and if seen, it might easily be mistaken for the 

 residence of some large insect. At other times they may be seen 

 in numbers on rocks in shallow water, basking in the sun, 

 apparently asleep. They bite severely when provoked, darting 

 forward with great velocity the long neck and head, and not 



