OPHIDIANS. 39 



extremity of a horizontal pedicle formed by the mastoid bone, 

 so connected as to allow of a certain yielding movement upon 

 the cranium. The other bones have similar loose movable articu- 

 lations, which concur in yielding to the pressure of large bodies 

 with which the teeth have grappled. 



The class of Heptiles is divided into three orders : — the Ophi- 

 dians, comprehending the Snakes ; the Saurians, the Lizards 

 and Crocodiles ; and the Chelonians, the Turtles and Tortoises. 



Ophidians. 



In Ophidians, commonly known under the name of Snakes, 

 the body is long, round, and straight. They have neither feet, 

 fins, nor other locomotive extremities. Their mouths are furnished 

 with pointed hooked teeth. In the Boas and Pythons the teeth 

 are slender, curved, bending backwards and inwards above their 

 base of attachment. In others each maxillary bone has a row of 

 larger ones, which gradually decrease in size as they are placed 

 further back. These teeth are not contiguous, being separated by 

 considerable intervals. The smaller non-venemous Serpents, such 

 as the Coluhridce, have two rows of teeth in the roof of the mouth. 

 Each maxillary and mandibular bone includes from twenty to 

 twenty-five teeth. In the Rattlesnakes and some other typical 

 genera of poisonous Snakes, the short maxillary bone only sup- 

 ports a single perforated fang. Their lower jaw is highly 

 distensible ; the ojiening being longer than the skull. They have 

 no neck ; their eyelids are immovable ; their skin is coriaceous, 

 highly extensible, and scaly or granulous, covered with a thin 

 caducous epidermis, which detaches itself in one entire piece, 

 and is reproduced several times in one year. Their movements 

 are supple and varied. In consequence of the sinuosity of their 

 bodies, — for, though scale- clad, Snakes are without apparent means 

 of progression, — they make their Avay with the utmost facilitj^ b}^ 

 walking, leaping, climbing, or swimming. 



According to the genus chiefl}", the very numerous species 

 inhabit either arid or moist places, the ground, or bushes and 

 trees. Some pass much of their time in the wat^er, and one 

 family (that of tho Ili/dropJiidce) is exclusively aquatic — even 



