196 
DR. MANTELL ON THE IGUANODON. 
last stage, when the crown is reduced to a mere plate of dentine, as in the specimen 
previously figured (Xylograph, No. 2, ante, p. 188)*. 
As the articular piece which contains the socket of the lower jaw for receiving the 
inferior head of the os quadratum is unfortunately wanting, the mechanism of the arti- 
culation can only be conjectured ; for although several examples of the tympanic bone 
— which in reptiles as in birds connects the lower with the upper maxilla — are pre- 
served in my former collection, neither of the specimens is sufficiently perfect to indi- 
cate the precise mechanism of the joint. One of the tympanic bones found in the 
same quarry with teeth and bones of the Iguanodon, and which I am led to consider 
as having belonged to that reptile, is 6 inches high and .5^ inches wide ; it consists of 
a thick pillar, which is contracted at the sides in its vertical direction, and has two 
thin expanded lateral processes ; it terminates both above and below in an elliptical 
and nearly flat surface; it is very cavernous from the large size of the tympanic cells. 
This bone differs considerably from the corresponding element in the Iguana, the 
peculiarity, doubtless, having relation to some modification in the mechanism of the 
lower jaw, by which the more complete comminution of vegetable substances was 
effected, than by maxillary organs constructed after the usual Saurian type. 
Physiological inferences. — In instituting a comparison between the maxillary organs 
above-described, and those of the existing herbivorous lizards, with a view of obtain- 
ing some physiological deductions from the peculiar osteological characters of the 
fossil remains, we are at once struck with their remarkable deviation from all known 
types in the class of reptiles. In the Amhlyrhynchi , the most exclusively vegetable 
feeders of the Saurian order, the alveolar process beset with teeth is continued round 
the front of the mouth; the junction of the two rami of the lower jaw at the sym- 
physis presenting no edentulous interval whatever, and the lips are not more deve- 
loped than in other reptiles : in the Iguanas, as shown in my former memoir, the 
same character exists. In the carnivorous Saurians the teeth are also continued to 
the syrnphysial suture on each side. The extinct colossal lizards offer no exception 
to this rule; in the acrodont Mosasaurus of the Chalk, and in the thecodont Megalo- 
saurus of the Oolite and Wealden, the jaws are armed with teeth to the anterior ex- 
tremity. In short, the edentulous, expanded, scoop-shaped, procumbent symphysis 
of the lower jaw of the Iguanodon has no parallel among either existing or fossil 
reptiles ; and we seek in vain for maxillary organs at all analogous, except among 
the herbivorous Mammalia. The nearest approach is to be found in certain Edentata 
— as for example in the Cholcepus didactylus or Two-toed Sloth, in which the anterior 
part of the lower jaw is edentulous and much-prolonged : the correspondence is still 
closer in the gigantic extinct Mylodons ; in which the symphysis resembles the blade 
of a spade used by turf-diggers, and has no traces of incisive sockets ; and were not 
this part of the jaw elevated vertically in front, and the rami confluent, it would pre- 
* For a detailed account of the microscopical structure of the teeth of the Iguanodon, see “ Odontography, 
or a Treatise on the Comparative Anatomy of the Teeth,” by Professor Owen, p. 246-253. 
