Ol 



FOSSIL MAMMALIA. 



might have been inferred, even in the absence of the teeth, from the structure of 

 the articular surface, or glenoid cavity for the reception of the condyle of the lower 

 jaw. As the ridge of enamel runs, as above described, in a direction approaching 

 that of the longitudinal axis of the skull, it is obvious that the grinding motions oi 

 the lower jaw should be in a proportionate degree in the transverse direction, I he 

 glenoid cavity, therefore, instead of being a longitudinal groove, and open behind 

 as in the true Rodents, is extended transversely, and is defended behind by a broad 

 descending bony process preventing the retraction of the jaw, and showing marks 

 of the forcible pressure to which it was subject. 



It is worthy of observation that, in the Wombat— which exhibits the Rodent 

 type of dentition, and, like the Toxodon, has remarkably curved molars, but in an 

 opposite direction— the condyle of the lower jaw is also extended transversely, and 

 adapted to an articular surface, which admits of lateral motion in the trituration 

 of the food. In the outward span of the zygomatic arches, in which Toxodon 

 deviates from the Rodentia, we may trace a relation of subordinacy to the above 

 structure of the grinding teeth and joint of the lower jaw : the widening of the 

 arches giving to the masseter muscles greater power of drawing the jaw from side 

 to side. The depth of the zygoma bespeaks the magnitude of these masticatory 

 muscles, and the included space shews that the temporal muscles were also 

 developed to a degree, which indicates the force with which the great incisors at 

 the extremity of the jaws, were used ; probably, like the canines of the Hippopo- 

 tamus, to divide or tear up by the roots the aquatic plants, growing on the banks 

 of the streams, which the Toxodon may have frequented. 



In the Rodentia, the zygoma, though sometimes as deep as in the Toxodon, is 

 generally almost straight, and the space included between it and the cranium is con- 

 sequently narrow. The zygoma also is placed more forwards in all true Rodents, 

 than in the Toxodon ; and, instead of abutting against the posterior alveoli, it ter- 

 minates opposite the anterior ones. It thus affords such an attachment to the mas- 

 seter, that this muscle extends obliquely backwards to its insertion in the lower jaw, 

 at an angle which enables it to act with more advantage in drawing forwards the 

 lower jaw,— a motion for which the joint is expressly adapted. In many Rodents, 

 also, there is a distinct muscle, or portion of the masseter, which passes through 

 the ant-orbital foramen, which is on that account of large size. In examining the 

 cranium of Toxodon, with reference to this structure, it was found that the ant- 

 orbital foramen was not larger than might have been expected to give transmission 

 to nerves requisite for supplying with sensibility the large lips, and whiskers with 

 which the expanded muzzle of this remarkable quadruped was probably fur- 

 nished. 



Having thus examined the cranium of the Toxodon in its relation, as a 



