324 



COMPARATIVE MORPHOLOGY OF VERTEBRATES 



noted in the reptiles. In the heart the right ventricle is crescent-shaparf in 

 horizontal section, and it tends to embrace the other. In both there is but 

 slight development of the muscular trabeculae. The conus and the sinus are 

 lost. The truncus is completely divided (fig. 348, A), the arch coming from 

 the left ventricle connecting with the right fourth arch, which alone reaches the 

 dorsal aorta in the adult, the left of the pair continuing only as far as the origin 

 of the subclavian of that side. There is the same disappearance of the post- 

 cardinals as in reptiles, while the iliac veins extend to the postcava (fig. 348, B). 



Fig. 346. Fig. 347. 



Fig. 346. — Heart of snapping turtle, Chelydra serpentina (Princeton, 479). ao, 

 aortic arch; c, coeliac artery; da, dorsal aorta; db, Botall's duct; ec, ic, external and in- 

 ternal carotids; la, left auricle; p, pulmonary artery; ra, right auricle; sc, subclavain 

 artery; v, ventricle; m, mesenteric artery. 



Fig. 347. — Posterior veins of alligator, after Gegenbaur. a, lateral abdominal; 

 av, advehent veins; c, caudal; i, ischiadic; il, iliac; k, kidney; I, entrance of lateral ab- 

 dominal into liver; pc, postcava; rv, revehent veins. 



The right jugular vein is larger than the left, which is sometimes completely 

 closed. 



MAMMALS. — In the mammals the two sides of th e heart ar e compl etely 

 sep arated, and the sinus iT merged in the right atrium, the sinuatrial valves 

 being rudimentary (valvulae Eustachii and Thebesi in the base of the postcava 

 and the coronary vein respectively) . In the monotremes both atrioventricular 

 valves consist of three cusps, but elsewhere the right is tricuspid, the left bi- 

 cuspid. The persistent left aortic arch forms the only connexion between the 

 heart and the dorsal aorta, and from its arch the subclavian and carotid arteries 



