VIL] THE EXTERNAL SKELETON. 269 



pared with those of the upper jaw. In the carnivorous type 

 modification results in a " sectorial " tooth (here, i.e. in the 

 lower jaw, a true molar), which may, as in the Badger, have, 

 in addition to man's five cusps, an anterior one and three 

 small posterior ones ; or only the extra one in front, as in 

 the Dog ; or finally, as in the Cat, this tooth may attain 

 its typical perfection. It does so by becoming, as it were, a 

 cutting blade, consisting of but two sharp cusps connected 

 by trenchant ridges, and biting against those of the upper 

 sectorial tooth like one of the blades of a pair of scissors 

 against the other. The posterior cusp seems to answer to 

 the front outer cusp of man, and the anterior cusp to the 

 extra anterior one of the Dog. 



In the type of dentition culminating in the Horse, we start 

 again from the transverse ridges of the Tapir ; we find double 

 crescents, as in the upper jaw, but with the direction of their 

 convexities reversed, in the Ox and Sheep tribe ; while in the 

 Horse we find a similar reversal, and the extra processes 

 springing from the concave aspect of the crescents. 



Other and different complexities of form, as in the Ele- 

 phant, will come more conveniently under the head of 

 " Structure." 



31. Below Mammals we meet with great varieties of form. 

 Thus, in certain Fishes (e.g. the Chetodons) the teeth are like 

 slender bristles. In the Efts the slender denticles terminate 

 in two points, and in three in the Fish Platax. 



In Lizards the teeth sometimes have serrated edges, as 

 in the Iguana; sometimes they are rounded, blunt, and 

 crushing. 



In Fishes we may meet with every variety of shape, and 

 sometimes, amongst the Sharks and Rays, a very great 

 variety in the same mouth. 



Teeth are sometimes excessively sharp and pointed, as in 

 Lamna and Odontaspis ; sometimes they are furnished with 

 singular processes, as in Goniadiis. 



Sometimes, as in the Australian Shark Cestracion, the 

 teeth in the front of the mouth are sharp and pointed, while 

 as we pass backwards they become flattened till they form a 

 sort of pavement to the jaws. 



It is the Rays, however, which present us with the most 

 wonderful pavement. In some of these Fishes (e.g. in 

 Myliobatis) the teeth are placed in close contiguity, like the 

 pieces of a mosaic, and, w r hat is most remarkable, long and 

 narrow teeth are placed in the middle line so as to cross 



