XXIII] 



CHELONIA 



595 



encircled with a bony ring and the ectopterygoid or transverse bone 

 is wanting. The palatal crest on the palatine is hardly perceptible. 

 There is but one temporal arcade which is chiefly composed of jugal 

 and quadratojugal bones. When the squamosal joins the postfrontal 

 as it^does in a few cases no fossa intervenes between this union and 

 the jugal-quadratojugal bar. Chelonia as already explained are 

 Monapsid reptiles and descended from a different stock from that 

 which gave rise to the other groups of Reptiles. In the marine 



21 



V & 



VII 10 



FIG. 295. Longitudinal vertical section through the cranium of a Green Turtle, 

 Chelone mydas x f . 



1. Parietal. 2. Squamosal. 3. Quadrate. 4. Basisphenoid. 5. Basi- 

 occipital. 6. Quadratojugal. 7. Pro-otic. 8. Opisthotic. 9. Pterygoid. 

 10. Palatine. 11. Bod passed into narial passage. 12. Exoccipital. 

 13. Epi-otic fused to supra-occipital. 14. Supra-occipital. ' 15. Pre- 

 maxilla. 16. Maxilla. 17. Jugal. 18. Postfrontal. 19. Vomer. 

 20. Prefrontal. 21. Frontal. v 1 and 2, vn, vm, ix, x, xi, xii. 



Foramina for the exit of cranial nerves. 



Chelone and its allies the postfrontal, squamosal, parietal and 

 quadratojugal coalesce to form a sheet of bone from the crest of the 

 skull to the lip, roofing over a cavity lying at the side of the cranium 

 and containing muscles. The parietals send downwards two vertical 

 plates which meet upward extensions of the pterygoids. 



Breathing is performed as in Amphibia, by a mylohyoid muscle 

 and other muscles causing movements of the hinder visceral arches, 

 of which there are three pairs. 



The heart in structure and mode of action resembles that of 

 Lizards and Snakes, the left-hand systemic arch conveying blood 



382 



