6 Journal New York Entomological Society, fVoi. xviii. 



A casual examination of this tabular arrangement reveals the 

 fact that about an equal number of genera are so far known from 

 Baltic amber and from Florissant. This total is, however, a very- 

 poor means of comparison, for the greatest diversity exists in the 

 representation of the individual families and groups. This is in 

 part readily accounted for by the different way in which the insects 

 have been entrapped previous to fossilization. As is well known, 

 only such species occur in amber as have come in contact with the 

 trunks of the trees bearing the sticky resin destined to become fos- 

 silized as amber. This at once exercised a selection with regard 

 to certain groups which normally frequent such places and would 

 appear to account for the presence of so many genera belonging to 

 the Bethylidse. Some such forms live in galls, others are parasitic 

 on wood-boring beetles, still others on leaf-hoppers, etc., which 

 would have brought them in proximity to the resin upon the trees. 

 Other forms like Mymaridae are so delicate and fragile that we can 

 scarcely hope ever to find their remains in petrified form, although 

 the beautiful preservation afforded by amber has made it possible to 

 identify many species imbedded in this medium. This family so 

 abundant in amber is, therefore, entirely absent in the Florissant 

 shales. Aside from the poorer preservation of the Florissant mate- 

 rial, the different way in which it has been laid down has resulted 

 in the selection of quite a different component of the then existing 

 fauna from that which appears in amber. The types occurring at 

 Florissant are almost exclusively actively flying forms or others 

 which live in proximity to bodies of water, since these deposits con- 

 tain the remains of insects which had either flown into the waters 

 of the original Florissant lake or one of its tributaries, or had possi- 

 bly been engulfed in volcanic ash with which they were transported 

 thither by streams. In some groups of Hymenoptera like the ants 

 (Wheeler, '08) this has resulted in mutually excluding from the 

 amber and florissant beds in great part that which occurs in the other. 

 Thus, chiefly the workers of arboreal species occur in amber, while 

 with few exceptions only winged forms are found in the Florissant 

 deposits. 



With the parasitic Hymenoptera, this is, however, not generally 

 the case in families like the Ichneumonidae, Braconidae, Evaniidae and 

 the Proctotrypoidea, as is shown by the contents of the foregoing 



