INTRODUCTION. 15 



ferred to genera not now extant. Granted tliat our knowledge of the sub- 

 ti'opical forms of this continent (with which as a whole at least our Floris- 

 sant fauna seems to be akin) is much too meager to be of great service ; 

 granted also that in many cases we are forced to establish new genera upon 

 what would be regarded among recent animals as too slender grounds : it 

 is nevertheless true that an unexpectedly large number of forms can not be 

 forced into modern genera already established ; in many cases, throughout 

 whole groups, kindred differences from modern types are found which in- 

 dicate considerable changes of structure in the intervening epochs along 

 parallel lines. In illustration of this we would call special attention to the 

 differences observed in the genera of plant-lice, and, in several places 

 among other Hemiptera as well as among the Coleoptera, to the decided dif- 

 ferences ill the relative lengtli of various members of the body. My own 

 belief, which springs from the comparisons instituted in the study of this 

 fauna, is that a much larger proportion of genera should really have been 

 founded, and that, for every type which may turn up in Central American 

 explorations of the near future identical with those now established upon 

 the fossils alone, it will be necessary to separate from the familiar surround- 

 ings in which 1 have placed it some other of the insects from the same beds. 



It should be stated that the larger part of the plates in this volume 

 were engraved before the insects were studied, except in a cursory manner 

 to separate the species; the insects are therefore not always properly 

 grouped, and tlie legends upon the plates are in part inaccurate. 



In the enumeration of the specimens at the end of the specific descrip- 

 tions the numbers of the obver.se and reverse of the same specimen are 

 always connected by " and " without any intervening comma, and this 

 typographical method is employed only for expressing this relation. 



In tlie study of these extinct insects much assistance has been received 

 from friends, to whom my cordial thanks are due; for valuable suggestions 

 from the late Dr. J. L. LeConte, from Baron R. von Osten Sacken, Edward 

 Burgess, Esq., and Drs. G. H. Horn and H. A. Hagen; for tlie open collec- 

 tions of the late G. D. Smith, Esq., and of Messrs. E. P. Austin and Samuel 



. . • .it' 



Henshaw; and for important aid in obtaining typical series of modern insects 



in various groups by Messrs. E. P. Austin, P. R. Uhler, E. P. Van Duzee, 

 Edward Burgess, Dr. A. Forel, and most especially Mr. Samuel Henshaw. 



