46 



THE CERATOPSIA. 



two and this median tooth lies immediately beneath the center of the tooth that is being dis- 

 placed. The other two displacing teeth lie one anterior and the other posterior to and a little 

 above the median, with the supero-anterior or supero-posterior border of their crowns pushing 

 against the opposing portions of the tooth being displaced. It thus happens that each tooth 

 during its passage upward in the jaw is assisting in pushing forward three teeth, while at the 

 same time being itself pushed upward by three other teeth.- As the small embryonic tooth, 

 with single root left widely open, at the bottom of the capacious pulp cavity moves upward 

 toward the alveolar border of the jaw the walls thicken, reducing the capacity of the pulp 

 cavity, the root becomes bifurcated through the pressure brought to bear by the crowns of 

 the teeth immediately beneath and on either side of it and it shifts from the vertical position 

 which it at first occupied in the jaw to the nearly horizontal position which it occupies just 

 before being shed by passing upward or downward in the jaw, as the case may be, along well- 

 defined grooves excavated in the surfaces of the bone within the dental chamber, each groove 

 describing the arc of a circle, as shown in fig. 47, and being opposed by a similar groove on the 

 surface of the bone forming the opposite wall of the dental chamber. 



Fig. 47.— Section of maxillary oi 

 Triceratops. showing interior 

 of dental magazine marked 

 with dental grooves. 



FUNCTIONS OF THE TEETH. 



In the Ceratopsia the lower jaw closes inside the upper, and the external borders of the func- 

 tional lower teeth oppose the internal borders of the superior teeth in such manner that the wear 

 i in either series takes place in a vertical plane. When the mouth is closed these surfaces oppose 



each other laterally, and in the process of oj>ening and closing the mouth 

 they act like the opposing blades of a pair of shears, the teeth function- 

 ing as cutting organs rather than as crushing or masticating organs. 

 The structure and arrangement of the teeth and jaws would seem to 

 indicate that these animals may have fed on grasses or the stems, 

 branches, leaves, and twigs of shrubs and trees, and that their food 

 was taken into the mouth in considerable quantities and probably 

 received first into large lateral pouches which lay along the external 

 sides of the maxillaries and dentaries and which the conformation of 

 these bones in this region would indicate were present. From these 

 lateral pouches the food could be readily passed between the jaws, 

 which would act on either side as double-bladed chopping knives, soon 

 reducing it to a condition suitable for its reception into the stomach. 

 The teeth functioned as cutting organs rather than as prehensile or crushing organs, and the 

 food was reduced by cutting into small bits rather than by crushing or grinding into a pulp. The 

 horny sheaths which doubtless incased the predentary and rostral bones would have been very 

 efficient as cropping organs. Their chief function would appear to have been the gathering of 

 suitable food, while at the same time being effective as offensive and defensive organs. 



THE TEETBBEAL COIUJIN. 



THE VERTEBRAL FORMULA. 



As nearly as can be determined the vertebral formula in Triceratops is as follows: Cervicals, 

 7 [8, R. S. L.];° dorsals, 14; sacrals, 10; caudals ?. There are no true lumbars. In giving the 

 vertebral formula as above I have considered as sacrals all those vertebrae in the sacral region that 

 are coossified by their centra. Some of these give no support to the ilia, and the anterior might 

 perhaps better be called a sacro-lumbar, since it is in fact but a modified lumbar, while the last 

 four are in reality sacro-caudals, representing the first four caudals which have become somewhat 

 modified and firmly attached to the sacrum. In the type of Triceratops brevicornus (No. 1834, 

 Yale Museum) the entire presacral series of vertebrae were found in position and interlocked by 

 their zygapophyses, but unfortunately just at the junction with the sacrum the vertebras and the 



a See footnote, p. 47. 



