468 Mabel Bishop 



Since Teras XX presents a normal, but double arterial supply, it is 

 not difficult to represent diagrammatically the transformations of the 

 primitive heart and arterial arches. Text figure 4, illustrating the normal 

 changes in a chelonian, is inserted for ready comparison. It differs 

 from the normal mammalian changes in two respects, first, both the right 

 and left fourth arches persist, second, the ductus Botalli functions per- 

 manently throughout the life of a chelonian. A comparison of this 

 diagram with text figure 5, which represents the underlying condition in 

 Teras XX, will show at once that in the latter each moietv is identical 



Text Fig. 4. Diagram showing the transformation in the primitive 

 arterial arches in a normal reptile. 



with that represented in the former, and that the communicating branch 

 between them may suggest a slight increase in the fourth arch in the 

 median region. Although in the diagram the hearts are rather widely 

 separated, it should be mentioned that in the specimen they are quite 

 close together, the sinus venosi being adjacent, and the heart of compo- 

 nent B has a slightly more anterior position than that of its fellow; a 

 line drawn across the ventral border of the auricles of the heart of A 

 would be on a level with the most posterior portion of the ventricle of 

 heart of B. 



