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986 ZOOLOGY. 
formidable tusks. Other important points of distinction are to be found in 
the particular relations of several bones of the skull, some of which strikingly 
resemble those found in the crocodiles. The two occipital condyles impart a 
batrachian feature not to be found in any other saurian. Various genera are 
ranged in this family by the German paleontologists: the more conspicuous 
of these are Mastodonsaurus and Labyrinthodon, the latter of which, in 
addition to a close set series of nearly equal teeth along both jaws, has 
another along the anterior part of the outer margin of each vomerine bone. 
Two or three canine-shaped teeth, much larger than the rest, are placed in 
the intermaxillary bones, and in each vomer; other tusks being somewhat 
irregularly implanted in other situations. So few fragments of the rest of 
the skeleton have been found that we cannot have a clear idea of the animal, 
which may, however, have resembled a frog in external appearance. Some 
species are known to have possessed an armor of bony plates like that of 
the crocodiles. Not the least interesting feature of the Labyrinthodonts 
is presented by the peculiar structure of the teeth, a transverse section of 
which exhibits the most complicated cerebriform convolutions and sinuosities 
of the cement and dentine. ‘The only parallel to this, among recent ani- 
mals, is to be found in some species of the North American ganoid fish, 
Lepidosieus or the gar. It is highly probable that the curious hand-like 
fossil foot-marks found in Scotland and Germany, and provisionally assigned 
to a Cheirotherium, were impressed by some labyrinthodont. No species — 
of the family have been found in North America. 
OrperR 4. CHELONIA. 
The order Chelonia, or tortoises, is readily distinguished from other rep- 
tiles by the entire absence of teeth and the immovable union of the ribs 
and sternum into a kind of box, within which the soft parts are inclosed, and 
from which project the head, tail, and four extremities. This box is 
always broader than high, sometimes nearly as broad as long, and consists 
of two portions: one superior, or dorsal, called the carapace, or buckler ; 
the other inferior, or ventral, known as the plastron or shield. The cara- 
pace is usually much arched, the degree of curvature varying with the 
family: the plastron is flat or sub-concave. The plastron usually consists 
of eight elements united in successive pairs, and a ninth occupying a notch 
between the first pair. In the carapace there are two sets of bony ele- 
ments, one margining the other. This solid framework is covered by horny 
plates of various shapes and sizes, not necessarily similar in this respect to 
the osseous elements; more rarely by a simple coriaceous integument. The 
extremities are covered by a scaly skin. There are additional peculi- 
arities in the osteology of the Chelonia, to which our limits forbid us to 
refer. 
The head, neck, and tail are movable, and often retractile within the 
shell; the eyelids are always three in number; no external auditory 
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