610 B. F. Kingsbury and H. D. Eeed. 



structures in different forms (Heterochronia) to appreciate that the 

 relative reversal of sequence in frog and salamander is not intrinsic- 

 ally important. Among the Urodela themselves we have an instance 

 of partial reversal, the operculum appearing early in Triton (about 

 18 mm.) and late in Amby stoma (at transformation). 



The opposite relation of the nervus facialis to the plectrum and 

 stilus columellse, is perhaps a more serious difference, since it is just 

 this criterion of relative position of sound-transmitting apparatus 

 and nerve that has been considered of decisive moment in the deter- 

 mination of homology. As has been stated above, Huxley introduced 

 confusion into the interpretation in Amphibia by faulty recognition 

 of indifferent connective tissue as the equivalent of the frog's plec- 

 trum, hence the course of the facial nerve came to be interpreted as 

 over the suspensorial-columellar ligament or stilus, whereas in all 

 forms examined just the opposite relation holds, — the facial nerve is 

 heloiv (cephalad or ventrad of) the columella, with three exceptions, — 

 ISTecturus, Proteus, and Typhlomolge, — and in these cases the 

 jugular branch alone passes above while the rami mandibulares, 

 internus and externiis, are below and in front of the columella. 



If the different relation of the facial nerve to the columella in 

 Urodela and Anura is prohibitive of the plectrum-columella homol- 

 ogy, there is necessary the recognition of three morphologically dis- 

 tinct squamoso-columellar connections in tailed amphibia, (1) in the 

 Proteida, (2) in Typhlomolge, (3) the remaining urodeles. This, 

 it is felt, is contrary to the ontogenetic evidence. There is involved 

 in this point the broader question of the value of the relation of 

 nerve to skeletal structure as a criterion of homology. The real 

 value of such a test of homology has already been questioned by one 

 of us (Kingsbury, '03, p. 333) and a few instances given of varia- 

 tion in the relation of nerve, ligament and muscle, — instances which 

 could, of course, be easily multipled many fold ; for example, in 

 the relation of the hyomandibular bone or cartilage to the hyoman- 

 dibular nerve, in fishes, etc. It was not deemed desirable to introduce 

 here further consideration of such evidence, as it would involve work 

 along broader comparative lines than was desirable at this time. Ad- 

 herence is given to the view entertained in the first contribution, — 



