WEALDEN FORMATIONS. 19 



the Mantellian Catalogue of the Fossils purchased by the British Museum, is assigned 

 to the Iguanodon. 



A reptile with vertebrae and ribs resembling in their chief characters those of the 

 amphicaelian Crocodiles, and with distinctive peculiarities, in which the Lacertians by 

 no means participate, might reasonably be conjectured to resemble the Crocodiles in 

 the form of the tympanic bone ; and if the reptile in question used its teeth for 

 masticating hard vegetable substances, we might with more reason expect that the 

 bony pillar, supporting the lower jaw, should be firmly and immoveably fixed through 

 its whole length, like the tympanic bone of the Crocodilians, and not be loosely 

 suspended to the skull by a single extremity, as in the Iguana and other Lacertians. 

 A very remarkable bone, discovered in the Tilgate strata, figured by Dr. Mantell in 

 the ' Geology of the South-east of England,' pi. ii. fig. 5, the resemblance of which to 

 the " os quadratum," or tympanic bone of birds, was first suggested by Dr. Hodgkin, 

 is assigned to the Iguanodon by Dr. Mantell. He describes it " as forming a 

 thick pillar or column, which is contracted in the middle, and terminates at both 

 extremities in an elliptical and nearly flat surface." In the Iguana and other reptiles 

 the lower end of the tympanic bone is terminated by a convex trochlea, which is 

 received into a corresponding cavity in the lower jaw ; and it may be asked : — Is the 

 modification of the bone in question, assuming it to belong to the Iguanodon, indicative 

 of a peculiarity of the joint of the lower jaw as remarkable as the structure of the teeth, 

 and correlated to their masticatory uses ? " Two lateral processes, or aleB^ Dr. Mantell 

 proceeds to state, "pass off obliquely, and are small in proportion to the size of the 

 column ; on placing these bones beside the os tympani of an Iguana, we - at once 

 perceive that the relative proportions of these parts are reversed ; for in the recent 

 animal the pillar is small and the lateral processes large. From the great size of the 

 body of the fossil, and the extreme thinness of its walls, the tympanic cellules must 

 have been of considerable magnitude, and have constituted a large portion of the 

 auditory cavities. PI. ii. fig. 1, (fig. 5 is meant,) accurately represents the most 

 perfect specimen in my cabinet ; it is 6 inches high, and 5^ inches wide at the longest 

 diameter of the extremity of the body. It exceeds in magnitude the corresponding 

 bone of the Mosasaurus, and is fourteen times as large as the same bone in an Iguana 

 4 feet long." Tab. X, p. 306. 



After a careful inspection of the specimen, as it now may be seen at the British 

 Museum, I have come to the conclusion that both extremities have been abraded or 

 fractured : and that the form of the articular surface is not unequivocally demonstrated 

 at either end. The parts described as "two lateral processes" appear to be the two 

 piers a , c, of the auditory arch of the tympanic, which arch is composed of a broad 

 thin plate of bone, and surrounds the " foramen auditorium externum," e, which is 

 of a narrow oval form. Although the shape of this bone indicates that it was much 

 less susceptible of movement than the tympanic bone usually is in Lacertian reptiles, 



