600 



CHORD ATA. 



Sub Order II. PYTHONOMORPHA. Large, extinct, extremely elon- 

 gate reptiles with four flipper-like limbs and strong swimming tail. 

 Flourished in the cretaceous. Mosasaurus, CUdastes. 



Sub Order III. OPHIDIA. The snakes are distinguished from most 

 lizards by the absence of limbs, and connected with this the similar verte- 

 brae in which only trunk and caudals can be distinguished. The caudals 

 lack ribs, but these are present and long in the trunk region, serving for 

 locomotion and supporting the body on their distal ends. . Since there are 

 legless lizards, it is further necessary to say that in the Ophidia the girdles 

 and sternum are lost, only the Peropoda having remnants of the hinder 

 appendages and pelvis, but these not connected with the vertebral column. 

 Further distinctions exist in sense organs and jaws. The columella is 

 indeed present, but tympanum and Eustachian tube are lacking. The eye- 

 lids also seem to be wanting, but examination shows, in front of the cornea 

 and separated from it by a lachrymal sac, a transparent membrane, com- 

 posed of the fused eyelids (outer cornea). The apparatus of the jaws (figs. 

 619, 627) is remarkable for its great extensibility, which enables snakes to 

 swallow animals larger than themselves, after coiling around them and 

 crushing them. This extensibility is in part due to the fact that the bones 

 of the lower jaw are bound together at the symphysis by elastic ligaments, 

 in part to the freedom of motion of the bones of the upper jaw (excepting 

 the small premaxillaries) and the palate. Further, the sqtiamosal (&g), 

 quadrate (Q), and transversum (Tr) are elongate and slender, the quadrate 

 being widely separated by the squamosal from the skull, while the zygo- 

 matic arch is entirely absent. The food is forced down the throat by 

 hook-shaped bones on palatines and pterygoids. A wide distension of 

 the stomach is rendered possible by the elasticity of its walls and the great 

 mobility of the ribs, which are not united ventrally by a sternum. 



In the non-poisonous snakes the dentition is similar on jaws and 

 palate bones (fig. 619). The vomer and, usually, the premaxilla are tooth- 



less. In the poisonous serpents poison 

 fangs appear on the maxilla (fig. 627) 

 and are distinguished from the other 

 teeth by their greater size and connex- 

 ion with a large poison ghuid. The 

 duct of the gland opens at the base of 

 the tooth ; the poison which is pressed 

 out by the pressure of the jaw muscles 

 is led to the tip of the tooth either by a 

 groove (proteroglyphic tooth, fig. 629, A) 

 or, when the groove is closed to a cnnal 

 (solenoglyphic tooth, B), through this 

 . pro- canal which opens at base and tip of 



teroglyphic (grooved) tooth of co- tl tnnth 

 bra, and section of same; #, #,, so- tue 1 tn - 



The asymmetrical character of the 



lungs is interesting. In the Peropoda 

 one lung (apparently the left) is much 

 smaller than the other ; in the poison snakes and some others it is rudi- 



Fm. 629 Pison fangs. 



lenoglyphic tooth (tubular) of rattle- 

 snake ; g, poison canal ; jo, pulp 

 cavity. 



