THE SLOTH 265 



tare. One does not see why the creature should require 

 teeth any more than the pangolins and ant-eaters which 

 also feed on termites. 



There are usually five teeth on either side of each jaw, 

 simple in shape, with crowns, which at first are rounded, 

 but soon wear flat. Careful inspection of the worn 

 surface shows a number of small holes which are the 

 apertures of as many canals, and the tooth seems to have 

 the structure of a cane. In fact, however, each tooth, 

 though apparently simple, is really composed of a closely 

 set bundle of very fine, long, cylindrical teeth, united 

 together side by side. As the reader no doubt knows, 

 each of our own teeth has a soft centre, known as 

 the "pulp," by the hardening of the outer part of 

 which each tooth has been formed. The very fine 

 canals, which run through the substance of each tooth 

 of the aard-vark, are the cavities for such pulp of the 

 various very long and slender teeth, by the fusion of 

 which each apparently single tooth is formed. Such a 

 structure exists in no other beast, and even in no other 

 reptile ; but, strange to say, the same thing is found in a 

 fish of the skate kind, known in science as " Myliobatis." 

 Yet the aard-vark can have no special relationship of 

 generic affinity with these fishes. 



It would be impossible to find a stronger instance of 

 that circumstance on which, in our previous articles, we 

 have so often insisted, namely, the independent origin of 

 similar structures. 



Such are the beasts that live on the earth's surface 

 to-day, and compose the singular, and singularly di- 

 versified, order of Edentates. 



Outside that order, we have already met* with two 

 animals which have been supposed to be therewith 

 * See pp. 49 and 50. 



