522 STUDIES FOR STUDENTS 
will be impossible to determine accurately their position with 
relation to the other reptiles. 
Oudenodon was also described from the skull alone; the jaws 
are entirely edentulous and the appearance of the skull is strik- 
ingly testudinate. It has been suggested that this is the female 
of Dicynodon. 
Ptychosiagum, or as it is more generally known, Ptychognathus, 
was very similar in many respects to the Dicynodon, having the 
same horny covering to the anterior portion of the jaws and the 
large canine teeth in the upper jaw; the peculiar part about the 
animal was the bending of the facial portion at almost a right 
angle to the upper surface of the skull and the upward extension 
of the anterior end of the lower jaw so that the mouth seemed 
to open on the superior face of the skull. 
All of these animals must have been herbivorous; the large 
canine teeth were probably used to tear up the roots and aquatic 
plants upon which they fed. They could have been of no use as 
organs of offense or defense. 
Placodontia.—These are rather problematical forms from the 
Muschelkalk (Triassic) of Germany. As in the case of the 
Dicynodontia the bones are all isolated and the various genera are 
known from the skulls; the few other bones that have been found 
in the same deposits may or may not belong with the skulls. 
Lydekker has called attention to the fact that all the bones that 
are found with the skulls are either dinosaurian or plesiosaurian ; 
if it should turn out that the plesiosaurian bones belong with the 
skulls of the Placodontia it may be that that group belongs very 
far from the Axomodontia with which they are here placed, but 
until that is established it is perhaps best to take the evidence 
of the skull which points to a very decided relationship between 
the two groups. /lacodus and Cyamodus are the two described 
genera. 
Placodus was first described by Agassiz from a single tooth, 
and was regarded by him as a fish. The error was not corrected 
until the entire skull of the form was known. The skull was 
triangular in outline, very broad behind and rapidly narrowing 
