THE ELASMOBRANCHS 69 



aging 250 cc. per kg. per day, as compared with 12 

 cc. or less in marine forms. The concentration of urea in 

 the blood of Pristis and in the other fresh-water elasmo- 

 branchs studied averages about 70 per cent below the 

 figure typically observed in marine forms; this reduction 

 appears to be related to the increased urine flow result- 

 ing from hfe in fresh water rather than to increased per- 

 meabihty of the gills. From these and other data one 

 can construct the cycle of events as Pristis moves up- 

 river. When it enters the brackish water, it is charged 

 with urea to the full extent, but it is in only a slightly 

 superior osmotic position in relation to its environment 

 —in other words it has only limited quantities of free 

 water available to it, and the urine flow is at a low level. 

 As it encounters water of decreasing salinity, its osmotic 

 position is improved and more water is absorbed through 

 the gills, with a consequent increase in urine flow; but 

 as the urine flow increases, the blood urea concentration 

 is decreased by urinary loss, until a new steady state is 

 reached. If it turns back to sea, the cycle is reversed and 

 decreasing availability of water decreases the urine flow 

 and increases the urea content of the blood imtil the 

 animal comes back into water balance. It is a complete 

 misreading of the record to say that the reduction of the 

 blood urea content in the fresh-water elasmobranchs is 

 a ^reversal of evolution': evolution has nothing to do with 

 Pristis or any elasmobranch swimming up and down a 

 tropical river— the problem is simply one of an automatic 

 physiological adjustment. 



After diversification in the Carboniferous, the elasmo- 

 branchs showed great reduction in number and variety 

 at the end of the Permian, a crisis that saw the extinction 

 of many ancient forms. Expansion occurred again in the 

 Triassic and Jurassic, and by the close of the latter 

 period nearly all the modem lines of sharks, rays, and 

 skates were represented in the seas. They siuvive today 

 as the oldest order of fishes above the cyclostomes, and 



