1046 BAnON FRANCIS NOPCSA ON REVEllSIULK 



of tlie anterior limb to the posterior is 10/27, in the Jurassic 

 Compsognaihtis 10/18, in the nearly contemporaneous Ornith- 

 lestes 10/15, and in the cretaceous StriUhioiiiimus 10/16. In 

 this group, in which not the jaws or the posterior limbs but 

 the anterior limbs were used for seizing the prey, a decided 

 lengthening of the anterior limbs occurs. In the heavily- 

 built carnivorous Dinosaurs, in which, much as in tlie birds 

 of prey, a prehensile foot is developed, this elongation does not 

 occur and the anterior limbs remain small or almost vanish (10). 



A relatively short anterior limb is also met with in all bipedal 

 orthopodous Dinosaurs. The ratio is 10/21 in Hypsilophodon, 

 10/23 in Thescelesaurus, 10/19 to 10/17 in Oamptoscntrtis, 10/14 

 in /(/uanodon, 10/17 in ICritoscmriis, and 10/15 in Corythoscmrus, 

 Though less clearly than the preceding one, this list also shows 

 that iir the more specialised forms, as Iguanodon and Corytho- 

 scmrus, the anterior limbs are a trifle longer than in the )nore 

 primitive forms. 



In the quadrupedal Sauropoda the anterior limbs are mostly 

 shorter than the posterior ; in one group, however, the Brachio- 

 sauridie, the length of the limbs is nearly equal. In this case 

 the secondary elongation is very marked (10). 



All these data show that in those specialised Dinosaurs in 

 which the anterior limb is continuously used a secondary 

 lengthening of this part occurs. This can be considered as a 

 reversal to the ancestral pro-dinosaurian type, l^iagrfunmatically 

 these changes can be expressed in the following manner: — 



Anterior limbs long and functioning. Anterior limbs short. 



Ancestors of Dinosaurs- 



"^Primitive Dinosaurs. 

 Specialised Dinosaurs ^ 



For one reason the case is not quite conclusive, for it can be 

 surmised thiit the a])pnrcnt secondary elongiitioii is not duo to a 

 renewed growth hut simply to tlie fact that in Dinosiiui'S the 

 posterior limbs, on account of their being more used, grew big 

 lirst, and that the growth of the anterior ones set in later. 

 8inoe also such a hypotliesis might explain the temporary dis- 

 proportion of the limbs, the case must be considered doubtful. 



§ (2) The development of the postorhital bar. 



The second case to be dealt with is more typical than the 

 first. While in all more or less pi'imitfve reptiles the postorhital 

 bar is complete, it is open in some rather specialised forms. 

 8uch Reptiles are : many snnke-shaped reptiles, some other 

 lacertilians, and the Bauriamorplia. Contrary to what is known 

 in Reptiles, in Mammals the postorhital bar is incomplete in the 

 primitive forms and complete only in tlie younger Equidre, most 

 of the Artiodactyla, and in tlie Primates. This being the case, 

 it may be concluded that all Mammals descended from animals 

 lacking a postorhital bar. 



