Page:Popular Science Monthly Volume 13.djvu/742

722 in common, separating them from the other divisions of the animal world. Their bodies are protected by modifications of the skin into scales, enormous rugosities of almost impenetrable horny plates or flat shields of various forms. No reptile ever has feathers, for, on account of a peculiarity of the circulation of the blood by which the aerated and unaërated portions mingle together, they are cold-blooded, and therefore do not require so heat-conserving a covering for the body. Most reptiles possess two pairs of legs, of which the fore limb conforms much more closely to the hind in structure than is the case between the anterior and posterior extremities of the bird. On these they crawl rather than walk, their bellies, which are dragged along the ground, assisting in the support of the body; some have both pairs adapted for aquatic life, while others are entirely devoid of progressional appendages. No member of the class can be called a true volant, notwithstanding that a few, such as the flying dragons of the Philippine Islands, are able, by means of membranous expansions of the skin, to sustain themselves in the air while passing from one tree or support to another. With the exception of the tortoises, the majority are carnivorous and possess powerful jaws set with strong, sharp teeth.

So much lies on the surface.

From an examination of the chief points of their internal framework we learn that the "collar-bones" do not unite to form a "merry-thought;" nor does the breastbone develop a median keel. In general the tail is more or less elongated, but its terminal segments do not unite to form a "ploughshare" bone. The leg-bone of the reptile differs from the bird's in having a well-developed fibula lying parallel to it throughout its whole length; it does not present a strongly-marked crest at its upper end, nor is the articular surface of the narrow lower extremity formed by the coalescence with the shaft of a separate bone into a pulley-shaped termination. The coalescence never takes place at all; but each retains a separate existence throughout life. In the situation of the hock in the bird the reptile has at least four distinct bones to which are articulated as many toes; and, lastly, the haunch-bone, instead of being a consolidated mass, is composed of two halves, one on each side, articulating with, but not united by, bony tissue to the spine, and meeting each other below—a character in which the struthious birds, such as the ostriches, agree. It may be remarked, also, that in their keelless breastbone, as well as in the disunion of their collar-bones, these birds present other similitudes to the reptiles.

Every student of osteology is well aware that all bones in their embryonic condition are composed of cartilage, wherein, as the animal grows older, bony spots or "centres" appear, whence the ossification spreads till the whole structure is converted into bone. Among the higher animals these centres are seen only during the earlier years of life, while with increasing age their outlines, becoming gradually fainter, are at length entirely lost. But among the reptiles many of