
APPENDIX B. 335 
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 correspond- 
ing 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, protruded successively in series of three, 
in the longitudiral 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 
C. arctatus, the section increases in size with protrusion ; hence, before the appear- 
ance of a new crown outside of it, there are but two functional teeth in 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 vertebra are episthocelian, 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 complex 
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 dentition is, by a system of complication 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 complica- 
tion. 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 ar- 
rangement, 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 
section, instead of two or three. 
C1onopon sTENopsis, Cope. 
This Dinosaur is represented by fragments of maxillary bones, with a few con- 
tained pieces of teeth. Probably, several of the numerous bones of reptiles of this 
order, obtained by Mr. Dawson, pertain to the same genus and species; but there are 
as yet no means by which to distinguish them from other species of I/adrosauride in 
the collection. 
The maxillaries exhibit the vertical grooves characteristic of the genus, and the 
teeth are of the rod-like form of those of the C. arctatus, Cope, and the roots are 
similarly compressed. An important difference in the teeth is seen in their lack of 
