60 



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

 The zygapophyses are but little prominent beyond the arch. A caudal verte- 

 bra is plano-concave, with rather depressed centrum, a little longer than broad. 

 The condyles of the femur have a short 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 vege- 

 table 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 dentition is, by a system of complica- 

 tion by repetition of parts, made to subserve an end identical with that 

 secured by deep plication of the crown of the more specialized molar of the 

 mammal. 



Cionodon is evidently allied to Hadrosaurus, 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 

 l here can be only one functional tooth in a transverse section, instead of two 



or three. 



Cionodon arctatus, Cope. 



Bulletin, /. c, p. 2. 



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

 the base of the shaft. Its margins are thickened and without serration, 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 tli>' alveolar groove 0.0120 



Length of a triad of teeth on the alveolus 0.0140 



Length of an unworn tooth 0.0^50 



Diameter of the surface of attrition of a tooth of the middle row : 



Longitudinal 0. 00C3 



Transverse 0. 007^ 



Width of the maxillary hone 0.0350 



Depth of t'ne maxillary bone at the inner margiu 0.0140 



