SAUROPSIDA. 257 
ing, or rudimentary, but in no case are the fore-limbs con- 
structed upon the type of the “ wing” of birds, and in no living 
Reptile is there the bone which is known in Birds as the 
“ tarso-metatarsus.” 
The class Reptilia includes the Tortoises and Turtles (Che- 
Jonia), the Snakes (Ophidia), the Lizards (Lacertzlia), and the 
Crocodiles (Crocodilia). With the exception of the Tortoises 
and Turtles, they are mostly of an elongated cylindrical form, 
furnished behind with a long tail. The limbs may be altogether 
absent or quite rudimentary, as in the Snakes, but in almost all 
the higher members of the class there are two pairs of limbs, 
which may be either adapted for walking or swimming, and 
which in some extinct forms support a flying membrane. The 
internal skeleton is always bony, never cartilaginous or semi- 
a. Sui 
SY 
WDDID WH} 
ee 
Fig. 138.—Skull of a Serpent (Python). a@ Quadrate bone; 4 Lower jaw, 
articulating with the movable quadrate bone. 
cartilaginous as in many of the fishes. The skull is joined to 
the spine by a single articulating surface (or condyle). The 
lower jaw is complex, each half being composed of several 
pieces united by sutures. In Tortoises and Turtles, however, 
these separate pieces are amalgamated together, and the two 
halves are also united, so that the whole lower jaw appears to 
form a single piece. In most reptiles, on the other hand, the 
two halves of the lower jaw (fig. 138), are only loosely united ; 
in the Snakes by ligaments and muscles, in the Lizards by gristle, 
and in the Crocodiles by suture. In all, the lower jaw is jointed 
to the skull by means of a special bone, called the “ quadrate 
bone ;” and as this often projects backwards, the opening of the 
mouth is often very extensive, and may even extend backwards 
R 
