Reptiles, whether Batrachians, Ophidians, or Chelonians, are mostly _oviparous_, sometimes _ovo-viviparous_, and some of them are very prolific. The eggs of some are covered with a calcareous envelope, as in the Turtle. Sometimes they are soft, and analogous to the spawn of fish, as in the Batrachians. They do not hatch their eggs by sitting upon them, but bury them in the sand, and take no further care of them, trusting to the heat of the sun, which hatches them in due course. To this the Pythons form a partial exception. Batrachians content themselves with diffusing their spawn or eggs in the marshy waters or ponds, or they bear them on their backs until the time of hatching approaches. On leaving the egg the young Tortoises have to provide immediately for their own wants, for the parents are not present to bring them their nourishment or to defend them against their enemies. This parental protection, so manifest among the superior animals, does not exist in oviparous species; that is, in those whose eggs are not hatched in the body of the mother. The young are, so to speak, produced in a living state, and fully prepared for the battle of life. The loves of these animals present none of that character of mutual affection and tender sympathy which distinguishes the Mammalia and Birds.[5] When they have ensured the perpetuity of their species, they separate, and betake themselves again to their solitary existence. [5] Birds, however, are oviparous, and nevertheless manifest the strongest parental affection.--ED. Some reptiles attain dimensions truly extraordinary, which render them at times very formidable. Turtles are met with which weigh as much as sixteen hundred pounds, and the carapace of one of these measured as much as six feet in length. The size of an ordinary Crocodile is from eight to nine feet, but they have been seen twenty-four and even thirty feet long, with a mouth opening from six to eight feet wide. [Illustration: Fig. 3.--Skeleton of a Turtle.] In Chelonians the surface of the skull is continuous without movable articulations. The head is oval in the Land Tortoises, the interval between the eyes large and convex, the opening of the nostrils large, the orbits round. The general distinguishing characteristic of Tortoises is the external position of the bones of the thorax, at once enveloping with a cuirass or buckler the muscular portion of the frame, and protecting the pelvis and shoulder bones. The ribs are inserted by means of sutures into these plates, and united with each other. A three-branched shoulder and cylindrical shoulder-blade are characteristic of the Tortoises.