CHAP. III THE THEROMORPHA QI 
eround by mentioning the fact that among the Theromorpha 
four distinct types of reptiles are included, which are considered 
to form four orders, 7.e. the Pareiasauri, the Theriodontia, the 
Anomodontia (Dicynodontia), and the Placodontia. 
The first of these divisions includes what seem to be basal 
forms. These reptiles show numerous points of likeness to the 
Amphibian Labyrinthodonts.' On the other hand the third divi- 
sion, that of the Dicynodontia, are highly-specialised Thero- 
morpha, from which no further evolution would appear to have 
been possible. Thus the dentition was either completely lost, 
or reduced to tusks as in Dicynodon. We need not therefore 
concern ourselves in the present volume with these Anomo- 
donts. It is with the Theriodonts that our business les. 
The very name, be it observed, is aptly chosen on the hypo- 
thesis to be explained here; but it is not only in the teeth 
that these reptiles show hkenesses to the Theria or Mammals, 
but in almost every feature of their organisation. Unlike other 
reptiles, the Theromorpha in general were lifted comparatively 
high above the ground on legs of fair length and of mam- 
malian relationship in the position of the segments of the 
limbs. The typical reptile grovels upon the earth with legs 
sprawling out, as indeed the very name suggests. One bar to 
the Theriodonts being on the direct line of mammalian ancestry 
has been urged as a preliminary difficulty, and that is their 
large size. The earliest undoubted mammals were small 
creatures, comparable to a Rat or a Mouse in size ; whereas a good- 
sized Bear or a Wolf is a better standard of size for some of the 
best-known genera of Theriodonts. It has, however, been quite 
permissibly suggested that ving in company with these large 
Theriodonts were less obtrusive genera, from which the mammals 
might have sprung. It is so familiar a fact that a given group 
of animals generally contains giants, dwarfs, and members of 
intermediate size, that this suggestion may almost be accepted 
as a fact. It need at least present no difficulties to us in our 
comparisons. 
The most salient “mammalian” feature of the Theriodonts 
is the heterodonty of the teeth, the pattern of the “ molars,” and 
the limited number which constitute the series. The fact, too, 
that they are limited to the dentary bones below and to the 
1Cf. vol: vilil.-p.. 82. 
