ON THE ORIGIN OF GENERA. (Ji 



natatoricB, wliicli is the homologue of the A. pulmonalis of air- 

 breathers, issues in Lepidosteus from the last vena IrancMcdis, 

 thus receiving aerated blood from the gills. In Lepidosiren it 

 issues from the point of junction of two gill-less and two gill-bear- 

 ing venoR hrancMales, thus receiving mixed blue and red blood, or 

 blue blood altogether, Avhen the branchiae are not in functional 

 activity. In Proteus it issues from the last vena branchialis, 

 where it receives the ductus hotalli of the preceding vein, which, 

 when the gill is inactive, becomes a gill-less aorta-bow, which brings 

 it only carbonized blood, which it readily aerates in the swim- 

 bladder, now become a lung. The ventricle is homologous with 

 the preceding. In salamanders, where the substitution of the 

 accessory gill arches by the ductus hotalli, converts the arterice 

 and vencB hrancMales into ^^ aorta-bows," the A. pulmonalis is 

 given off from the posterior bow, and receives henceforth mixed 

 blood. In the Anura the origin is the same, but nearer the heart. 

 In CaeciliidaB it approaches the heart so far as to issue from the 

 extremity of the hulhus arteriosus, which is now divided by an 

 incomplete septum, one half conveying blood to the aorta-roots, 

 and the other to the A. pulmonalis. This septum was already 

 preceded by a longitudinal valve with free margin in the Anura ! 

 As if to meet the coming event, a trace of ventricular septum ap- 

 pears at the apex within. There can now be no question of the 

 homology of the ventricles of the gar, and of the Cascilia. But 

 we have next the true Eeptilia. The hulhus arteriosus is split 

 externally, as it already was internally, but it is first represented 

 in most tortoises by an adherent portion, one half being the now, 

 to this point, independent arteria pulmonalis and the other the 

 nearly sjjlit aorta-roots. There can, I think, be little question of 

 the exactitude of the homology throughout. 



It is no less certain that the salamander * fulfills in its devel- 

 opment the different stages to its permanent one, and is identical 

 in each stage, in respect to this point, with the orders it represents 

 at the time. This is true even of the long period during which 

 it bears the long branchial appendages and contained arteries and 

 veins which are not found in fishes ; it is then like the Protop- 

 terus, which has hyoid venous arches and appendages of those 

 arches at the same time. The tortoise f and Troj^idonotus I are 

 also identical in their successive stages with the types already 



* Amblystoma. f Agassiz. X Rathke. 



