208 THE ANATOMY OF YERTEBEATED ANIMALS. 



They may be developed in the premaxillaries, maxillaries, 

 palatines, pterygoids, and the dentary piece of the mandible, 

 but their presence in the premaxillaries is exceptional. In 

 JTtopeltis and some other genera, there are no palatine teeth ; 

 and in the egg-eating African snake, Hachiodon^ the teeth 

 are small and rudimentary upon all the bones which usually 

 bear them. But the inferior spines of eight or nine of the 

 anterior vertebrse are long, and tipped, at their apices, with a 

 dense enamel-like substance. These project through the 

 dorsal wall of the oesophagus into its cavity, and the eggs, 

 which are swallowed whole, are thus broken in a position in 

 which all their contents must necessarily be saved. 



In the majority of the non-venomous Snakes the teeth are 

 simply conical, but in the others, and in all the poisonous 

 Snakes, some of the maxillary teeth (which are usually longer 

 than the rest) become grooved in front. In the Solenoglyphia^ 

 or Vipers and Rattlesnakes, the maxillary teeth are reduced 

 to two or three long fangs, the groove in the front of which 

 is converted into a canal open at each end, by the meeting of 

 its edges. The teeth of the Snakes are replaced by others 

 which are developed close to the bases of the old ones. 



Ophidia are not known in the fossil state before the older 

 tertiaries. 



V. The IcHTHYOSATJRiA. — In its general form Ichthyo- 

 saurus presents a good deal of resemblance to a Cetacean. 

 The head is enormous, and passes at once into the trunk, so 

 that there is no more appearance of a neck than in a Porpoise, 

 and the body tapers off behind, much as would happen in the 

 latter animal were it devoid of a caudal fin. Indeed, there is 

 some reason to suspect that the tail of Ichthyosaurus may 

 have been provided with a sort of fin-like expansion of the 

 integument. This fish-like body was propelled, like that of 

 the Hesiosaurus^ by four paddles ; but the anterior paddles 

 were placed close behind the head, and were, generally, very 

 much larger than the posterior ones. 



Pig. 75.— a restoration oi IchihyoBamnia. The existence of the caudal fin is doubtftil 



