296 PALEONTOLOGY. 



The Cetiosaurus has the bones spongy like those of cetacea, 

 with no medullary cavity in the long bones ; this genus, 

 it is conjectured, attained the size of our largest whales. Of 

 the four species known, one is from the Portland stage, and 

 the others are from the wealden of England. 



2nd Family. The MEGALOSATJEIKID^. This family is 

 formed of extinct reptiles, remarkable for their gigantic 

 dimensions, "and for possessing a combination of anatomical 

 characters which are found in no living genus of this class, 

 by which the reptilian type of structure is connected with 

 the mammalian, and a transition established between the 

 crocodiles and the lizards. They resemble the mammalia 

 in the form and size of the bones of their extremities, in 

 which a well-developed medullary cavity is excavated. Their 

 feet were short and resembled those of the pachyderms. 

 The sacrum was formed of five vertebrae soldered together 

 as in mammals; whilst in all other reptiles, living and 

 fossil, the sacrum is composed only of one or two bones. 

 The teeth of some resembled those of crocodiles, whilst those 

 of others were allied to lizards. This family comprises 

 three genera. 



The Megalosaurus had a straight muzzle, flattened on the 

 sides, like the dolphin of the Ganges. The 

 teeth were compressed and sabre-shaped, 

 and finely denticulated on the edges. 

 (Fig. 200.) The mode by which they 

 were fixed in the jaw is intermediate be- 

 tween the crocodiles and the lizards. They 

 were attached to the external wall of the 

 maxillary bones in distinct sockets ; internal 

 to the row in use were others in different 

 stages of development, destined to replace 

 FIG. 200. those that were lost through decay or 

 accident. This disposition of the teeth 

 siate. i s analogous to the dentition of many exist- 



ing lizards. The remains of this genus are found in the 

 great oolite of England and Normandy, also in the three 

 stages of the wealden in England. 



The Hylceosaurus is from the wealden of Tilgate Forest. 

 The Iguanodon is allied to the Iguana by the form of the 

 teeth : they are not f.xed in distinct sockets, but are soldered 



