CONTRIBUTIONS TO THE KNOWLEDGE OF THE REPTILES 
OF THE KARROO FORMATION. 
By Dr. E. C. H. van TIoepen, M.I. 
2. The Lower Jaw of Lystrosaurus . 
While describing* the skull of Lystrosaurus Putterilli many 
difficulties were encountered through the fact that our knowledge of the 
lower jaw of Lystrosaurus is still in a very unsatisfactory state. It was 
necessary before proceeding airy further with this fossil, especially its 
lower jaw, of which only two disconnected portions are visible, to 
study the elements of this part and their exact relations in other 
specimens . 
The following is a review of what had been made known before 
on this subject. I could not go further back than 1855, because 
Owen’s previous work could not be obtained. 
Owen only mentions the dentary in the description of Dicynodon 
tigriceps ( 1 ). A low, obtuse, longitudinal ridge projects from this 
bone, corresponding with the one on the premaxillary. There are 
two ridges separated by a groove on the upper border of the left 
ramus. The groove gradually deepens backwards (for so far present). 
There is another ridge on the outside of the rami and a vacuity further 
backwards. 
Huxley adds to this in 1859 that there are three ridges on the 
symphysis, which fit into grooves on the palatal surface of the 
premaxillary. These ridges are separated from each other by deep 
grooves. The upper border of the dentary is, broad, and is formed by 
a thick inner and a thin outer wall, separated by a groove. The 
thick inner wall becomes thinner downwards and reaches the opercular. 
The surangular is covered by the outer wall. The opercular elements 
are united in the symphysis. 
Owen contributes further to the knowledge of this subject in 
1860 in the description of Lystrosaurus latirostris ( 3 , p. 53) : — The 
lower jaw is continued backwards for a small distance behind the 
articulation with the quadrate. The articulation surface is hollow. 
It was impossible to distinguish the angular, surangular, and articular 
from each other. Angular and surangular diverge forwards. The 
angular forms the hind and lower boundary of the vacuity in the 
outside of the jaw. The fore part of the vacuity is formed by a 
bifurcation of the dentary element. The angular is wedged in between 
the opercular and the dentary in the symphysial region. The ridge 
on the outside of the dentary subsides forwards at the vertical channel 
upon the side of the symphysis, which receives the corresponding tusk 
when the mouth is shut. 
In 1862, while describing L. Alfredi ( 4 , p. 458), Owen states that 
lie is still unable to distinguish the articular from the surangular. 
There is no coronoidal process and no complementary. The longi- 
tudinal ridge on the outside of the dentary parallels that formed by 
the canine alveolus of the maxillary. 
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