580 UNGULATES. 



two lines and a half thick and were entire and imperforate. The 

 water percolating the stratum in which this tooth had lain, had 

 found access to the cavity through the porous texture of its walls, 

 and had deposited on its interior a thin ferruginous crust, but the 

 cavity had evidently been the result of some inflammatory and 

 ulcerative process in the original formative pulp of the tooth, very 

 analogous to the disease called ' spina ventosa' in bone. I have 

 given two figures of this singular case of primaeval disease in my 

 " History of British Fossil Mammalia." 



212. Succession. — The practical inducements to pay attention 

 to the times of cutting and shedding the teeth have operated more 

 strongly in the case of the Horse than in that of the domestic 

 Ruminants, and have led to more numerous and persevering re- 

 searches on the development and succession of the teeth. The 

 deciduous formula of the genus Equus is : — 



. 3-.3 1-1 4-4 



m. — ; c. — ; m. — -: 62. 



3-3 ' 1—1 ' 4-4 



The second molar (' first grinder' of Veterinary Authors, d m, 

 fig. 4, PI. 136) is the first to pierce the gum: the white summits of 

 the ridges of the crown are usually apparent at birth, but sometimes 

 the gums do not yield until from the second to the fifth day ; the third 

 molar (' second grinder', fig. 5, d 3) rises a day or two later, often 

 simultaneously with the proceeding: their appearance is speedily 

 followed by that of the first incisor (' centre nipper', fig. 4, di 1), 

 which usually cuts the gum between the third and sixth days. The 

 second incisor {di 2) appears between the twentieth and fortieth 

 days, and about this time the first small deciduous premolar 

 (fig. 4, p. 1) takes its place, and the fourth deciduous molar 

 (' third grinder', fig. 5, d 4) also, begins to cut the gum. About 

 the sixth month the inferior lateral or third incisors, (fig. 4, di 3) 

 make their appearance together with the small deciduous canine 

 (fig. 4, d c.) This minute tooth is shed, in the lower jaw at least, 

 almost as soon as the crown of the contiguous incisor is in full 

 place, being carried out by the same movement ; whence the small 

 canine had almost escaped notice until Bojanus drew the attention 

 of veterinary authors to it by his memoir, 'De dentibus caninis 



