274 Proceedings of the Royal Society of Victoria. 



cases (95 per cent.) have the greatest thickness (12), in Hyla aurea 

 we find that the central thickness (12) of the range has the 

 greatest number of instances (40 per cent.), with a gradual 

 increase in percentage up to this, and then a gradual decrease. 



It will be noticed that both nerves IX. and III. in Hyla aurea 

 reach a greater thickness than any other nerve in one case in 

 each nerve, and also that the 3 or 4 thicknesses below these 

 extremes are not met with. 



Nerve II. In both Bufo variabilis and Hyla aurea the great 

 majority of instances have the smallest thickness but one, being 

 in the former, thickness 5, in the latter, thickness 4. 



It has been found from observations on the larva; of several 

 of the Amira,! that in the tail of the larva are several pairs of 

 spinal nerves which disappear during metamorphosis. Between 

 this and the adult, intervene stages in which nerves X., XL and 

 XII., have in succession the following thicknesses 9, 4, 1; 8, 2, 0; 

 7, 1, 0, and sometimes 6, 0, 0. But since we must regard the 

 larval condition as similar to that of the ancestors of that 

 animal, the first of the above four stages is the most primitive, 

 and the last the most highly advanced. From this reduction of 

 the most posterior spinal nerves in order, we can deduce the most 

 primitive and the most advanced condition of spinal nerves 

 anterior to these. For this purpose it is necessary to proceed 

 from these most posterior nerves to those in front of them. 



The following Tables, B, 1 to 28, show all the combinations of 

 thicknesses found in those specimens of Hyla aurea examined. 



Tables B, 1 to 8 show the various thicknesses with which 

 nerve XII. occurs, but owing to the few specimens containing 

 examples of this nerve we cannot rely too much on results 

 obtained from them, unless as confirming other results. 



1. While XII. decreases, XL increases. Thickness 1 of nerve 

 XII. is found with the higher thicknesses 3 and 5 of nerve XL, 

 while thickness 2 of XII. is only found with thickness 2 of 

 nerve XL 



2. While XII. decreases, X. increases. Nerve XII., thickness 

 1, is only found with the higher thicknesses 6, 7, 8, 9, of nerve 

 X. ; and nerve XII., thickness 2, with nerve X., thickness 5. 



; Adolphi : loc. cit., 1S93, p. 335. 



