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A MANUAL OF DENTAL ANATOMY. 



of the ramus, those at its posterior end being materially 

 smaller; and the sockets are deeper 

 and stronger than in the upper jaw. 

 The succession takes place vertically, 

 as in Crocodiles and Dinosaurs. 



The genus Hesperornis, probably 

 diving birds, includes species 6 feet in 

 length : as has already been mentioned 

 the teeth are not implanted in distinct 

 sockets, but lie in a continuous groove 

 like those of Ichthyosaurus ; slight pro- 

 jections from the lateral walls indicate 

 a partitioning off into sockets, but 

 nothing more than this is attained, and 

 after the perishing of the soft parts the 

 teeth were easily displaced, and had 

 often fallen out of the jaws. The pre- 

 maxillary is edentulous, but the teeth 

 extend quite to the anterior extremity 

 of the lower jaw : in one specimen 

 there are fourteen sockets in the max- 

 illary bone, and thirty-three in the 

 corresponding lower ramus. 



The successional tooth germs were 

 formed at the side of the base of the 

 old ones, and causing absorption of the 

 old roots, migrated into the excavations 

 so formed, grew large, and ultimately expelled their prede- 

 cessors, as is seen in the accompanying figure. 



In structure these teeth consist of hard dentine, invested 

 with a rather thin layer of enamel, and having a large axial pulp 

 cavity. The basal portion of the roots consists of osteodentine. 



( ] ) Mandible of Ichthyornis (after Prof. Marsh). A. Side view, show- 

 ing the teeth in situ. B. View of upper surface, showing the sockets in 

 which the teeth were implanted. 



