342 



TERTIAEY INSECTS OF NORTH AMERICA. 



follows I have omitted all consideration of that, to make the comparisons 

 move equable. For the same reason, in order to use the last work at all, I 

 liave instituted comparisons only between the families there elaborated, and 

 have used the family groups in the same sense as there, except only that 

 I have regarded his Pyrrhocoridre as a group of Lygseidee. 



These four families are indeed the very ones, and, as will be seen, the 

 only ones which assume any importance in the American Tertiaries ; and 

 a comparison of their interrelation as to numbers can be shown succinctly 

 by the following table, which exhibits the relative percentage of representa- 

 tion of each of these families in the different regions and times as represented 

 in the published lists — the only available ones, and which may be supposed 

 to represent, not the numbers, but the relations with tolerable accuracy : 



The correspondence of the numbers in the last two columns is even 

 less remarkable than the disturbance of the relative percentages of the Cap- 

 sidse and Lygjeidaj of the western list wdien compared with those of the 

 American and Central American forms ; the merest indication of such an 

 overturn is shown in the comparison of the nearer American and the more 

 distant Central American lists ; but the overturn is still more complete and 

 in the same direction when we compare the existing and the fossil fauna of 

 the West. The relative representation, then, of the four principal families 

 of the Tertiary Heteroptera of the western half of our continent agrees con- 

 spicuously better with the relative representation of the existing fauna of the 

 same geographical region than with that of the other regions of the same 

 world. Either the physical conditions of the region in question have 

 remained since Oligocene times in the same relative contrast to those of the 

 other regions under comparison, or the present Heteropterous fauna of the 

 West shows a decided relation to that which existed on the same ground in 

 Tertiary times, or both. 



