250 NATURAL SCIENCE. April. 



sponding milk-molars so as to show the form of the functional end 

 before cutting the gum. In these specimens, therefore, there are four 

 rows of upright teeth between the jaws of the right side, the two 

 middle rows being the teeth in use at the time, and the upper and 

 lower rows the premolars which have not yet cut the gum, an arrange- 

 ment clearly comprehended by a glance at the accompanying photo- 

 graph of one of the preparations. 



In the case of the incisor and canine teeth of the left side another 

 method of exposition has had to be adopted. The teeth, being 

 curved, are set rising obliquely from the board so as to present their 

 biting surfaces. Those permanent incisors which have not yet cut 

 the gum are placed behind, i.e., to the left of, the corresponding 

 deciduous incisors in use at the time. 



All the teeth exhibited are lettered in a manner readily intelli- 

 gible to those possessing but the slightest knowledge of odontology, 

 and the labels, though large enough to be legible to the inquiring 

 student, are so small as to be unobtrusive. The collection is, more- 

 over, fortunate in having the case in which it is exhibited specially 

 constructed for it, most museum exhibits having usually to accommo- 

 date themselves to show-cases of stereotyped pattern and size. 



" The periodical changes to which the teeth of the horse are 

 subject, afford reliable indications of age, second only in value to 

 positive evidence of the date of the animal's birth. The ages of 

 cattle, sheep and swine are to be judged with accuracy only during 

 the period occupied by the cutting of the temporary teeth and their 

 replacement by those of the permanent set, but the peculiar confor- 

 mation of the teeth of the horse enables the expert to form an opinion 

 of the animal's age long after the completion of the permanent denti- 

 tion." (Brown, Jouvn. Roy. Agric. Soc, 1881.) The specimens 

 exhibited in the Natural History Museum not only show the age at 

 which each tooth is formed and when it cuts the gum, but they also 

 show those changes which the crowns of the permanent teeth 

 undergo with advancing age, upon which the practical horseman 

 relies in his determination of the age, when the date of birth is either 

 not forthcoming or is considered untrustworthy. 



The horse possesses that number of teeth which may be con- 

 sidered normal for Mammalia, and these teeth are divided into groups 

 according to their form, position, and other characters ; namely, three 

 incisors, one canine, four premolars, and three molars on each side of 

 each jaw — forty-four teeth in all. 



The cheek teeth (molars and premolars) are set in close contact, 

 as are also the incisors ; but between the canine and the first pre- 

 molar there is a broad gap (diastema), in which the " bit " of the 

 driving gear of the horse is placed. This diastema, however, appears 

 comparatively late in development ; for at five months before birth 

 the first deciduous molar (dm 2, the first premolar having no 

 predecessor) is in close proximity to the second deciduous incisor, the 



