59 



supports a keel in the middle, thus giving rise to two shallow longitudinal 

 troughs. The remainder of the tooth is covered with a layer of some dense 

 substance, possibly cementum, 

 which overlaps the vanishing 

 margins of the enamel. The 

 outer inferior excavation of the 

 shaft presents a median longitud- 

 inal groove to accommodate the 

 keel of the closely-appressed 

 crown of the successional tooth. 

 The apex of the tooth being ob- 

 tusely wedge-shaped, the func- 

 tional tooth is pushed forward 

 and transversely toward the inner 

 side of the jaw. The tooth slides 



downward in a closely-fitting Ver- Fl <3- I.— Diagram of the maxillary dentition of Cionodon 



arctatus: a, grin ding-face ; 6, superior or radical view of 

 tical groove of the OUter alveolar the maxillary bone, natural size. 



wall. The inner wall is oblique, its section forming with that of the outer a V; 

 it is furrowed with grooves similar and opposite to those of the outer wall, but 

 entirely disconnected from them. The base of the shank of the functional 

 tooth, on being displaced by the successional, slides downward and inward 

 along the groove of the inner side; each lateral movement being accompanied 

 by a corresponding protrusion. At the most, three teeth form a transverse 

 line ; namely, one new apex external, one half-worn crown median, and the 

 stump or basis of a shank on the inner. The new crowns are, however, pro- 

 truded successively in series of three in the longitudinal direction also. Thus, 

 when an apex is freshly protruded, the shank in front of it is a little more 

 prominent, and the third stands beyond the alveolar border. As each shank 

 increases somewhat iu diameter downward in the C. arctatus, the section 

 increases in size with protrusion ; hence, before the appearance of a new 

 crown outside of it, there are but two functional teeth iu a cross-row. Thus, 

 in the outer longitudinal row, only every third tooth is worn by functional use 

 at one time ; in the middle series, all are in use ; while, in the inner, every third 

 one is simultaneously thrown out in the form of a minute stump of the shank, 

 if not entirely ground up. 



The dorsal vertebrse are opisthocoelian, the anterior more compressed 



