On the Classification of the Reptilia. 



273 



Sauropsidan branch. There seems to be no escape from this conclusion ; 

 nevertheless, it does not solve the difficult problem of the true nature of the 

 roofing of the skull. For it may still be held that the Chelonia branched off 

 from the base of the Sauropsidan stem when the heart and metatarsal had 

 become specialised but the roofing had not yet been pierced. On the whole, 



artery ; r, connecting region which" remains open only in Sphenodon and certain 

 Lacertilia ; ra, right auricle ; rsa, right systemic arch ; rv, right ventricle ; 

 sc, subclavian ; spa, iuterauricular septum ; spv, interventricular septum ; si; sinus 

 venosu3 ; v, ventricle ; vc, vena cava superior ; vci, vena cava inferior. Arrows 

 from the sinus venosus indicate the main stream of venous blood ; arrows with 

 a dotted line indicate the stream of arterial blood from the left auricle ; 1-6, the 

 original series of six aortic arches. 



it seems more likely that the roofing of the skull has been secondarily 

 restored, but the discovery of fossil intermediate forms alone can settle this 

 question. 



Lacertilia. — There is much difference of opinion as to the position of the 

 order Lacertilia (including the Pythonomorpha and Dolichosauria). Some, 



