286 ZOOLOCT. 



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 

 rano-ed in this family by the German palasontologists : the more conspicuous 

 of these are Mastodonsaurus and Lahyrinthodon, 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 

 irreo-ularly 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, 

 Avhich 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, 

 Lepidosceus 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 Cheirotherhan, were impressed by some labyrinthodont. No species 

 of the family have been found in North America. 



Order 4. Chelonia. 



The order Chelo?iia, 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 Avhich 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 mucl» arched, the degree of curvature varying with the 

 family : the plastron is flat or sub-conca-ve. 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 

 490 



