22 



posite to tliose 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 grove of the inner 

 side, each lateral movement being accompanied by a corresponding pro- 

 trusion. 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, protruded 

 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 in diameter downward, in the (J. arc- 

 tatus, the section increases in size with protrusion ; hence, before the 

 appearance of a new crown outside of it, there are but two new func- 

 tional teeth in a cross row. Thus, in the outer longitudinal row, only 

 every third tooth is in 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 vertebrpe are opisthocoelian, the anterior more compressed 

 than the posterior; capitular articular faces, if existing, are slightly 

 marked. The zygapophyses are but little prominent beyond the arch. 

 A caudal vertebra is plano-concave, with rather depressed centrum a 

 little longer than broad. The condyles of the femur have ashort arc 

 and chord ; the head of the tibia .displays a large cnemial crest, but is 

 not emarginate behind. 



The type of dentition exhibited by this genus is perhaps the most com- 

 plex known among reptiles, and is well adapted for the comminution of 

 vegetable food. While the mechanical effect is quite similar to that 

 obtained by the structure of the molars of ruminating mammals, the 

 mode of construction is entirely altered by the materials at hand. Thus 

 the peculiarly simple form and rapid replacement of the reptilian denti- 

 tion, is by a system of complication by repetition of parts, made to sub- 

 serve an end identical with that secured by dui)lication of the crown of 

 the more specialized molar of the mammal. 



Cionodon is evidently allied to Hadroscmrus, but displays greater 

 dental complication. In that genus, according to Leidy, the successional 

 crowns appear on the front side of the shank of the tooth, not behind, 

 and below the base of the enamel area, so that the tooth is distinguished 

 into crown and shaft. It also follows from this arrangement that the 

 successional tooth does not appear until its predecessor has been worn 

 to the root, in which case there can be only one functional tooth in a 

 transverse direction instead of two or three. 



Cionodon aectatus. Cope. 



Bulletin, 1. c, p. 10. 



Char, specif. The enamel plate of the tooth extends from apex to near 

 the base of the shaft. Its margins are thickened and without serra- 

 tion, while the surface generally is nearly smooth. The dense layer over 

 the remainder of the tooth is much roughened by a great number of 

 short, serrate, and somewhat irregular longitudinal ridges. 



Measurements. 



M. 



Width of alveolar groove 0.0120 



Length of a triad of teeth on alveolus 0140 



Length of an unworn tooth 0250 



