ix TRIONYCHOIDEA 407 



Islands, and to Africa from the Nile to the Senegal and to the 

 Congo. 



The habits of Trionychoidea have found few observers. Accord- 

 to L. Agassiz, 1 they live in the muddy bottom of shallow waters, 

 burying themselves in the soft mud, with only the head, or a 

 small part of it, exposed. They breathe without moving the 

 body, by raising up the long neck and carrying the leathery snout' 

 above water. When moving through the water they strike 

 horizontally with both pairs of limbs, alternating, however, the 

 right and left ; but when they start suddenly, the front limbs are 

 seen moving together towards the tip of the snout, and then 

 striking simultaneously backward with great power. As the 

 shield does not project forward, the fore-limbs usually move 

 beyond the shield, and as its outer edge is sharp, and the feet are 

 broad, their webs reach above as well as below the plane of that 

 edge, so that the water is driven partly over and partly under 

 it. When they move along the bottom, the limbs still move 

 horizontally, the webs striking against the water, and the inner 

 toes, those with the claws, against the bottom. They also bury 

 themselves horizontally, becoming covered by only a thin layer of 

 mud. They readily resort to the shell for protection. The neck 

 and head are withdrawn entirely, the loose skin rolling off from the 

 greater part of the neck ; and the skin of the legs also slips off, as 

 far as the elbows and knees. In confinement they exhibit great 

 quickness ; their movements are abrupt and unsteady, except when 

 they swim rapidly in one direction. They then dart their long 

 and slender neck quickly forwards or sideways and upwards, as 

 snakes do, and bite in the same way, striking suddenly. Their 

 temper is bad or even ferocious, and large specimens are quite 

 dangerous. 



Their food consists of all sorts of aquatic animals, fish, frogs, and 

 molluscs, for instance Anodonta and Pcdudina. According to the 

 different diet, many species develop a peculiar kind of dimorph- 

 ism, a reasonable explanation of which has been given by 

 Boulenger. In the young the horny coverings of the jaws are 

 sharp, with cutting edges, and in those specimens which keep to 

 a diet of fish and other soft creatures, the jaws remain in the 

 same condition. But in those which take to living upon molluscs, 

 the hard shells of which they have to crush, the horny edges are 



1 Contributions to the Natural History of the U.S.A., vol. i. 1857, p. 333. 



