Digestive System Special. 77 



crowns. In the great frugivorous species (Pteropidac) the Molars 

 have broad flat crowns. 



Rodentia. The Eodentia are characterised by two large and long 

 curved Incisors in each jaw, separated by a wide interval from the 

 Molars ; for no Rodent is possessed of Canines. Structure. The 

 Incisors consist of a body of compact Dentine, more dense anteriorly 

 than posteriorly, with a plate of Enamel laid generally upon the 

 anterior surface of the tooth alone (the Enamel consisting of two 

 layers, of which the external is denser than the internal) ; and a 

 general investment of Cement, which is very thin upon the Enamel. 

 In Leporidse the Enamel is traceable to the back of the tooth. The 

 Molars have always a coating of Enamel, the vertical folds of which 

 generally penetrate the crown of the tooth : these folds have a general 

 tendency to a transverse direction. In those Rodentia that have 

 rootless Molars the folds of Enamel extend inwards for the entire 

 length of the sides of the tooth, which may be thus distinguished 

 from the rootless Molar of the In-Enamellata. The Chinchillidae, 

 and Capybara, aiford good examples of the Dentes * compositi,' with 

 plates of Enamel, followed by Cement, parallel. In all Rodentia, 

 vascular Dentine forms but a small constituent of the Molar tooth ; 

 the pulp, after forming a certain thickness of tubular Dentine, 

 becomes converted into Osteodentine. Replacement.^' The first or 

 anterior tooth of the Molar series, whether the number be 2-2, 3-3, or 

 4-4, is a Premolar {i.e., has displaced a deciduous predecessor in a 

 vertical direction). Number. The Incisors (= Dentes Scalprarii), 

 are never more than two in number in either jaw, except in the 

 Leporidse, which have two pair, placed one pair (the smaller) behind 

 the., other, in the upper jaw. The Molars are not numerous in any 

 Rodent. Situation. The Molar teeth are obliquely planted, the 

 series on each side converging anteriorly in both jaws. Form. The 

 upper pair of Incisors describe a larger segment of a small circle, 

 the lower ones a smaller segment of a larger circle : they are of 

 unlimited growth {i.e. with ^ persistent^ pulps), and therefore without 

 'neck' or 'fang:' they have a sharp anterior margin, from which 

 they slope obliquely back, like a chisel, and are thence named dentes 

 scalprarii: when an opposing incisor is lost, following the curve 

 prescribed to their growth by the form of their socket, their points 

 often return against some part of the head, completing the circle. 



* Cf, Marsupialia, id, infra. 



