50 



TEETIAEY INSECTS OF NORTH AMEEICA. 



Here it appears at a glance that exactly the same groups are represented 

 in the stratified deposits of Europe and America in every instance, except- 

 ing the Dysderides, which is unrepresented in Europe and has a single 

 member in America. It also appears that only those groups which are 

 represented abundantly in amber (and all of them) are also represented to 

 some extent in the American fauna and (excepting, as before, the Dysder- 

 ides) in the European rocks. Exception should perhaps be made for the 

 European amber genus Archrea, the position of which in the Laterigradse is 

 uncertain, and of which Thorell says: "This genus may perhaps for the 

 present best be taken as the type of a separate family" of Laterigradse 

 (European spiders, p. 232). Six species are known, and they are classed 

 above as uncertain. The relation brought out in this table is certainly 

 striking, but it should be noticed at the same time that the Drassides and 

 Theridides, and especially the latter, are enormously represented in the Bal- 

 tic amber, and in comparison with them (though not by any means to the 

 same extent in comparison with the other groups) feebly represented in the 

 stratified deposits of Europe and America. 



We may venture one further investigation, although little weight can be 

 given to it from the meagerness of the data, viz, a comparison of the per- 

 centage of representation of the different larger groups in the different 

 horizons of Tertiary times in Europe with that of Florissant, where all the 

 American species so far known have been found. 



Percentage of groups of Tertiary spiders of Florissant, Colorado, compared with those of Europe. 



As this table shows so great a difference between the percentage of 

 representation in the Oligocene and Lower Miocene of Europe that it can 

 scarcely prove very instructive, it still seems to indicate a greater difference 

 between the Florissant deposits and those of Oeningen than between the 



