THE THEROMORPHA pi 



ground by mentioning tlie fact tliat among the Theromorplia 

 four distinct types of reptiles are included, which are considered 

 to form four orders, i.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 lies. 

 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 likenesses 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 Eat 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 living 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 



1 Cf. vol. viii. p. 82. 



