No. 377.1 THE ORIGIN OF THE MAMMALIA. 317 
the type of Thrinaxodon, which lacked the incisor teeth. ‘One 
of the principal features in common is the structure of the 
palate, resembling that of the Mammalia in the opening of 
the palato-nares between the molars, 
Seeley distinguishes the theriodonts from the dicynodonts by 
the following characters: The postorbital arch is similar, but in 
the theriodonts the malar bone has a greater backward exten- 
sion, and in the dicynodonts the squamosal has a greater 
downward development, the latter difference being due to se 
degeneration of the quadrate in the theriodonts. Dicynodon, 
moreover, has a tripartite condyle, as in the Chelonia (composed 
equally of basi- and exoccipitals); while the theriodonts have 
paired condyles, as in 
the Mammalia (1895, 
5, p. 129), with a de- 
pressed __ basioccipital 
portion. Both types 
show mammalian anal- 
ogies in the palate, as 
well as a fixed and re- 
duced quadrate. 
In the palate of Di- 
cynodon the palatine 
bones are separated by 
e TS ahe- Thee the eiaha e, p paes ig 5 
dian line. The occiput 
is broad and flat, bounded by the parietals and interparietals - 
above, there being a deep notch in the median line. The 
bones doubtfully described by Owen as paroccipitals (“ opis- 
thotics ” of Huxley) are fused with the exoccipitals, as observed 
by Huxley in Ptychognathus and by Seeley in other forms. 
Laterally, the occiput is formed by the sguamosals, elements 
which are very extensively developed in Dicynodon, largely 
covering the quadrate and descending to form nearly half of 
the glenoid facet for the lower jaw, a very important character. 
In this respect this genus is more mammalian than the theri- 
odonts, in which the squamosal does not form part of the 
glenoid facet. 
a px the equamésal upon the occiput. The bones 
