256 NATURAL SCIENCE. April, 



shed, yet it not infrequently remains to coexist for many years 

 with the other three premolars, more especially if it is situated 

 so far forward in the jaw as to be unaffected by the eruption of 

 the second premolar. The first premolar is so variable in its occur- 

 rence that it is not considered as a normal tooth by veterinarians, 

 but is regarded as a supernumerary premolar, and goes by the 

 name of the " wolf-tooth." Modern investigation tends to show that 

 the alveolus and dental follicle of the first premolar are developed 

 in all cases at some five or six months before birth, and larger in the 

 upper jaw than in the lower ; but that in some cases the alveolus 

 closes without ever containing a calcified tooth, while in others the 

 tooth is absorbed without cutting the gum. On account of the wide- 

 spread belief that these wolf-teeth exert, in some mysterious way, an 

 injurious influence on the eyesight of the horse, they are often forcibly 

 removed at the instigation of the owner ; but, as the roots are short 

 and single, and as the teeth are of little service for mastication on 

 account of their small size, no great harm is done by this blind 

 adherence to a superstitious custom. 



The cheek teeth (second, third, and fourth premolars and the three 

 molars) are massive, and remarkable for their great length of crown, 

 the bulk of which is embedded in the alveolus in the young animal, 

 but which is gradually pushed outwards as the table or grinding 

 surface is worn away. The lower cheek teeth are provided with 

 two roots, and the upper with three. The roots begin to form at four 

 or five years, but indications of their development may be seen shortly 

 after the tooth cuts the gum. They continue to grow in length until 

 they close at twelve or thirteen years, after which no more enamel 

 and dentine are formed, but the roots become coated with a gradually 

 increasing layer of cement (Fig. E.). The parts of the crowns above 

 the gum are in very intimate contact with one another, and appear to 

 form a single solid block ; but exposure of the embedded portions of 

 the teeth shows that the roots of the different teeth diverge to a con- 

 siderable extent. The cheek teeth of the right and left sides are more 

 widely separated in the upper jaw than in the lower, and, instead of 

 biting with their opponents by level surfaces, meet them obliquely, 

 the internal border of the lower teeth being higher than the external, 

 while the reverse is the case in the upper jaw. 



The cheek teeth are prismatic in form, and the last molar is 

 distinctly curved. The external face of the second upper premolar is 

 marked by three shallow vertical grooves and two intermediary ridges, 

 while that of the remaining upper cheek teeth is traversed by two 

 grooves separated by a ridge. The internal face of the upper cheek 

 teeth is marked by a very prominent ridge, the accessory column or 

 pillar of the tooth (a.p., Fig. A.). The sides of the lower teeth are 

 also marked by ridges and grooves, but not to the same extent as in 

 the upper teeth. The second premolar tooth is triangular in section, 

 but the remaining teeth are rectangular in section, and are remark- 



