COPE.J PALEONTOLOGY — CRETACEOUS PEEIOD VERTEBRATA. 449 



appeamnce of a new crowu 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 vertebrae 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 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 

 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 duplication 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 there can be only one functional tooth in a 

 transverse direction instead of two or three. 



Cionodon arctatus, Cope, Bulletin, loc. cit, 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 

 seriation, 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 p;roove 0120 



Length of a triad of teeth on alveolus 0140 



Length of an unworn tooth 0250 



Diameter of surface of attrition of a tooth of the middle row, (longitudinal) 0063 



Diameter of surface of attrition of a tooth of the middle row, (transverse) 0072 



Width of maxillary bone 03r>0 



Depth of maxillary ji,t inner margin 0140 



What I suppose to be the posterior end of the maxillary bone exhibits 

 the grooves to near its apex, as well as a considerable surface of articu- 

 lation for the malar. 



Two dorsal vertebrae are preserved, whose neural arches are co-ossified, 

 with trace of suture remaining. Both articular faces exhibit a trans- 

 verse fossa for ligamentous or bursary attachment. Eound these, on the 

 convex face, there are transverse rugosities, while oblique-ridged lines 

 descend on each vside from the iloor of the neural canal. The centra are 

 shorter than deep, and subquadrate in a horizontal section. The sides 

 29 GS 



