722 THE POPULAR SCIENCE MONTHLY. 



ters 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 

 unaerated portions mingle together, they are cold-blooded, and there- 

 fore 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 an- 

 terior 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 frame- 

 work we learn that the " collar-bones " do not unite to form a " merry- 

 thought ; " nor does the breastbone develop a median keel. In gen- 

 eral 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 dif- 

 fers 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 situa- 

 tion 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 tw r o 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 



