ISio.] 503 



. and much more important step towards the Mammalian type of 

 dentition by maintaining- the serviceable state of the tusk by virtue 

 of constant renovation of the substance of one and the same matrix, 

 according to the principle manifested in the long-lived and ever- 

 growing tusks and scalpriform incisors of the Mammalia. This 

 endowment of the teeth of a reptile is far more remarkable and 

 unexpected than the more obvious character of the size and shape 

 of the long exserted tusks themselves, superadded as they are, and 

 in such strange combination, with the otherwise edentulous jaws 

 of a bird or turtle. Yet if we consider the fact in its relations to 

 the exigencies and convenience of the living animal, the wisdom 

 and beneficence of the principle is apparent, and the departure 

 from the ordinary rule manifests a power transcending the 

 trammels of scientific system. The teeth of the Dicynodon being 

 (but two in number, and their use to the animal indicated by their 

 'unusual size to be of unusual importance, the inconvenience and 

 detriment that must have ensued from frequent shedding and re- 

 placement is very obvious ; we may readily conceive it to have 

 been incompatible with their functions, and therefore abrogated in 

 favour of another mode of renovation which is abnormal in rep- 

 tiles, simply, pei'haps, because the form, proportions, and function 

 jof such tusks were unique, and are now no longer manifested in a 

 |cold-blooded class. 



j Some observations may be naturally expected in reference to 

 the probable use of the tusks to the Dicynodons, and the mode of 

 llife of those ancient and most remarkable saurians. In the Mam- 

 imalian class, where alone we now find the analogous instruments, 

 (tusks are usually given as weapons of offence and defence, — an 

 |o{fice exemplified in the hornless musk-deer, the boar, and in the 

 darge canine teeth of the Carnivora, The elephants use their tusks 

 lehiefly, though not exclusively, as lethal weapons : the Walrus is 

 -said to apply his tusks to aid in clambering over icebergs, as well 

 as in combat and defence : the Dugong is supposed to wear the 

 exserted points of the tusks in detaching fuci for food. Such an 

 office at first suggests itself as a very probable one in regard to 

 tusks descending, like those of the Dugong, from the upper jaw, 

 and combined with edentulous and probably horny mandibles 

 like those of a fucivorous turtle. 



I On inspecting the remains and the impressions of the tusks in the 

 ; fossils under consideration, and especially in the almost entire skull 

 I of one species, the Dicynodon lacerticeps, we perceive that these 

 weapons are sharp-pointed, and present no trace of that obliquely 

 bevelled or chisel-shaped extremity which is produced by habitual 

 application in acts of obtaining daily food, as, for example, in the 

 j protruded extremities of the tusks of the Dugong and the incisors of 

 I the Rodents. The tusks of the Dicynodon, though similar, in their 

 origin from maxillary bones and downward direction, to the tusks 

 of the Walrus, are so much shorter, at least in the single specimen 

 in which their entire length is shown, that they could not be avail- 

 able in locomotion. I conclude therefore from their shape, pro- 



TOL. IV. TAKT HI. P P 



