794 STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 



be regarded as more especially the champion of the second 

 view. 



One of us held in a recent publication 1 that the question was 

 not yet settled, though the view that the ribs are homologous 

 throughout the series was provisionally accepted. 



It is admitted by both Gegenbaur and Gotte that in Lepido- 

 stens the ribs, in the transition from the trunk to the tail, bend 

 inwards, and finally unite in the region of the tail to form the 

 ventral parts of the haemal arches, and our researches have 

 abundantly confirmed this conclusion. 



Are the haemal arches, the ventral parts of which are thus 

 formed by the coalescence of the ribs, homologous with the 

 haemal arches in Elasmobranchii ? The researches recorded in 

 the preceding pages appear to us to demonstrate in a conclusive 

 manner that they are so. 



The development of the haemal arches in the tail in these two 

 groups is practically identical ; they are formed in both as simple 

 elongations of the primitive haemal processes, which meet below 

 the caudal vein. In the adult there is an apparent difference 

 between them, arising from the fact that in Lepidosteus the 

 peripheral parts of the haemal processes are only articulated with 

 the basal portions, and not, as in Elasmobranchii, continuous 

 with them. This difference does not, however, exist in the early 

 larva, since in the larval Lepidosteus the haemal arches of the tail 

 are unsegmented cartilaginous arches, as they permanently are 

 in Elasmobranchii. If, however, the homology between the 

 haemal arches of the two types should still be doubted, the fact 

 that in both types the haemal arches are similarly modified to 

 support the fin-rays of the ventral lobe of the caudal fin, while in 

 neither type are they modified to support the anal fin, may 

 be pointed out as a very strong argument in confirmation of 

 their homology. 



The demonstration of the homology of the haemal arches of 

 the tail in Lepidosteus and Elasmobranchii might at first sight be 

 taken as a conclusive argument in favour of Gotte's view, that 

 the ribs of Elasmobranchii are not homologous with those of 

 Ganoidei. This view is mainly supported by two facts : 



1 Comparative Embryology, Vol. II., pp. 462, 463 [the original edition]. 



