80 Mammalia. Digestive System. 



as long tusks in the upper jaw, one in each premaxillary bone. 

 The Molars are large and complex : of these there are never more 

 than two in place or in use on each side at any given time. Struc- 

 ture. The Incisors, here called tusks, have * persistent pulps/ and 

 consist of ivory, i.e. a modification of dentine, with exceedingly small 

 dentinal tubes, arranged in almost angular gyrations, and which 

 shows, in section, strise, proceeding in the arc of a circle^ from the 

 centre to the circumference in opposite directions, and forming by 

 their decussations curvilinear lozenges ; the ivory is lowly vascular.- 

 The Molars afford striking examples of dentes lamellosi^' ; the 

 growth of each plate begins at the summit, consequently they remain 

 as separate denticles until their bases become contiguous and blend 

 together. The number of plates in the first Molar is four, and each 

 succeeding tooth has on the average four more plates than the one 

 immediately preceding it. Replacement. The Molar teeth succeed 

 each other from behind, moving in the arc of a circle. Numher. 



2.2 6.6 



The total number of teeth developed appears to be i - - . m ^ - 



JPerissodactyla. The Incisors and Canines are near together, and 

 a long diastema intervenes between them and the Molar series. 

 The crown of from one to three of the hinder Premolars is as com- 

 plex as those of the Molars. The outer side of the Upper Molars 

 is generally impressed by two wide longitudinal channels. 



The Incisor teeth of the Equidae are distinguishable from those of the Ru- 

 minants by their greater curvature ; and from those of all other animals by the 

 fold of Enamel which penetrates the body of the crown fi-om its broad flat sum- 

 mit, like the inverted finger of a glove, followed externally, as usual, by the 

 cement, constituting the * mark 'f of veterinary authors. The Canines are large 

 in the Horse; rudimentary in the Mare. The lower Canine, though situated 

 as in the Ruminants, close to the outer Incisor, is distinguished by its cuspidate 

 form. The Molars of the Equidae may be distinguished from the complex teeth 

 of other Herbivora corresponding with them in size by the great length of 

 the tooth before it divides into fangs. In the Tapiridse, the existence of a 

 transverse without an antero -posterior valley results in the division of the 

 crowns of the Molar series into a pair of transverse ridges. 



* Cf. Capybara (Rodentia), supra. 



t The cavity in the lower Incisors, or ' mark,* is usually obliterated, in the 

 first or mid-Incisors at the sixth year, in the second Incisors at the seventh year, 

 and in the third or outer Incisors at the eighth year. 



