Digestive System Special. 81 



Hyracoidea. The Incisors are large, and, to a certain extent, Eo- 



dent-shaped : the Canines are wanting : the Molar series is of the 



* typical' number. The first Premolar differs from the rest only by 



a graduated inferiority of size, which in the last Premolar, p m 4, 



ceases to be a distinction between it and the true Molars. The 



pattern of the upper Molars is unsymmetrical. 



1.1 0.0 4.4 3.3 



Dental formula : i r- ; c - ~ ; p m ^ - ; m -~ = 34. 



Artiodactyla. The crowns of the Premolars are smaller and less 



complex than those of the true Molars, usually representing half of 



3.3 ^ ^ 

 such a crown. The true Molars are constant, m ~ r ; the Pre- 



o . o 



molars are subject to variety. 



Omnivora. Structure. The Canines generally have Enamel only on the 

 anterior convex surface. The tusks of the Hippopotamus exhibit the maximum 

 density in their component tissues : the compact dentine differs from true 

 ivory by showing in transverse section simple concentric, instead of curvilinear 

 decussating lines. The last true Molar of Phacochserus is perhaps the most 

 peculiar and complex tooth in the whole class of Mammalia : the surface of the 

 crown presents three series of enamel islands, in the direction of the long axis 

 of the grinding surface, blended into a coherent crown by abundant cement. 

 Number. Sus. Scrofa has the typical dental formula. Form. Excessive de- 

 velopment of Canines characterises all Omnivora : in Hippopotamus the Inci- 

 sors are affected in the same way ; the two median inferior incisive tusks being 

 cylindrical, and of great size and length. The Molars of Hippopotamus are 

 characterised by the trefoil shape of the two divisions of the crown, one valley 

 bisecting the middle of the crown transversely, with two shorter indents parallel 

 to it, one on either side. In the Suidae the upper Incisors decrease in size from 

 the first (i 1) to the third {i 3), receding from each other in the same degree : and 

 the teeth of the Molar series progressively increase in size from the first to the 

 last. In Babyrussa the upper Canines are bent abruptly upwards, and curve 

 backwards, penetrating the upper lip. 



liuminantia. The development of horns bears an inverse relationship to the 

 development of the anterior teeth. The typical Euminantia (Cavicornua) never 

 have upper Incisors (their place is taken by a callous pad), and very rarely 

 upper Canines : their inferior Canines are procumbent, and are similar to, and form 

 part of the same series with the Incisors : most of their deciduous Molars re- 

 semble in form the true Molars, which are separated by a wide interval from 

 the Canines, and have their grinding surface marked by two double crescents, 

 the convexity of which is turned inward in the upper and outward in the lower 

 jaw. The last true Molar of the lower jaw in all Ruminantia is" characterised 

 ])y the addition of a third posterior lobe : and in all Ruminantia the outer con- 

 tour of the entire Molar series is shghtly zigzag, the anterior and outer angle of 

 one tooth, projecting beyond the posterior and outer angle of the next in 



M 



