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Mr. E. S. Goodrich. 



one broad bar, and maintains that it is a little modified descendant of the 

 ancestral lizard stock. Believing that the Lacertilia have been derived from 

 such Synapsidan forms, he would separate them from the Khynehocephalian 

 and other Diapsidan orders. According, however, to Lortet's description and 

 figures (21), one of which is reproduced here (fig. 3, E), the fifth metatarsal 

 would appear to be of normal shape and size. If this interpretation is correct 

 the evidence is distinctly against the view that Pleurosaurus is closely related 

 to the ancestral lizard. 



The Significance of the Heart and Aortic Arches in the Phytogeny of Reptiles. 



Turning to the evidence to be derived from the structure of the heart and 

 aortic vessels we shall find that it points unmistakably to the very same 

 conclusion reached from a study of the hind foot, namely, that all the modern 

 Eeptilia have been derived from a single Sauropsidan branch, distinct from 

 that which led to the Synapsidan Eeptilia and the Mammalia. 



It is well known that the heart of all reptiles (excepting the Crocodilia) is 

 possessed of a single ventricle, that the cavity of this ventricle is incom- 

 pletely subdivided by an incipient septum which is only completed in the 

 Crocodiles, and that the four-chambered heart of the latter is essentially like 

 that of a bird. In the possession of two completely separated ventricular 

 chambers the heart of a bird resembles that of a mammal ; and it is commonly 

 stated that the two groups differ in that whereas in the former the aortic arch 

 remains on the right side, in the mammal it is the left aortic arch which 

 alone persists. But the difference is far more fundamental than such a 

 statement implies. 



The original aortic system, as shown by a comparison of the fish and the 

 embryonic stages of the Tetrapods, consisted of six paired aortic arches. The 

 first supplied the mandibular bar, the second the hyoid, and the remaining 

 four the branchial bars. The last of these arches, the sixth of the original 

 series, gives rise to the pulmonary artery. The heart itself consisted of a 

 series of chambers : the posterior sinus venosus receiving the great veins ; 

 the atrium, which in air-breathing vertebrates becomes separated into two 

 auricles ; and a ventricle passing forward into a ventral aorta. This trunk 

 becomes divided into a posterior contractile conus arteriosus, or bulbus 

 cordis, and an anterior non-contractile truncus arteriosus, from which spring 

 the aortic arches. The heart becomes twisted, so that the auricles come 

 to lie dorsally and in front of the ventricle ; but in the accompanying 

 diagrams (fig. 4) of the heart and arches of an Amphibian (A), a Mammal (B), 

 a Beptile (Lacertilian, Ophidian, Bhynchocephalian, or Chelonian) (C), and a 



