280 Proceedings of the Royal Society of Victoria. 



IV. accompany the greater thicknesses of nerves IX., VIII., 

 and III.; it is not easy at tirst sight to understand why in 

 Hyla aurea while the sacral plexus agrees entirely with that of 

 Bufo variabilis, the brachial plexus should show such a marked 

 unconformity with the European forms. The true bearing of 

 this difference in nerve IV., will, however, appear later on in 

 this paper. 



Of the other spinal nerves, V., VI., and VII., have a fairly 

 constant and similar thickness, viz., chiefly 3, 4 and 5, the 

 greatest percentage in each having a thickness of 4. 



In order to trace the influence of this change in the thickness 

 of the spinal nerves, on their form and position, I have summed 

 up my observations on the relative positions of the nerves 

 forming the two plexuses in a series of figures and tables. 

 Fig. 1 to 4, and Tables C to L relating to the sacral plexus, and 

 Fig. 5 and 6, and Tables M to O to the brachial plexus. 



Fig. 1 and Table C show the gradual development by means 

 of which the function of nerve IX. in giving rise to the ileo- 

 hypogastric nerve has been passed on to nerve VIII. 



jrho 



Looking at Table C and remembering the results which we 

 obtained from the previous set of tables, that the smaller thick- 

 nesses of nerve VIII. are the more primitive, and the highest the 

 most advanced — we see that without any question form iii. is 

 the most advanced, for it not only is the sole one occurring 

 with all the thicknesses from the lowest to the highest, but it 

 gradually increases in frequency with an increasing thickness of 

 nerve VIII., until with thicknesses 5 to 8 it reaches 100 per 



