the species, of course, are supposed to agree with living ones; but most of the genera agree well, except in certain groups, like the plant lice, in which there is a similar divergence throughout.
Owing to this singular constancy of fundamental structure, we are able to ascertain that some striking types now confined to particular parts of the world, were once very widely spread.
Perhaps the most remarkable instance of this sort is afforded by the tsetse fly, Glossina. Scudder obtained from Florissant an imperfect specimen of a large fly, which he regarded as representing a new genus and species. We were so fortunate as to secure a much better example, showing the long proboscis, and it was not difficult to recognize it as a veritable tsetse fly. The species, of course, is extinct; but the genus is the same as that now confined to Africa, where it is dreaded as the disseminator of some of the most terrible diseases known. What part the existence of such flies may have played in the destruction of the Tertiary mammalia we can only surmise; but it is not impossible that their influence was great. How it happened that they disappeared entirely from America and survived only on the Ethiopian continent is, of course, unknown.
Another discovery, hardly less interesting, was a species of Neuroptera belonging to the family Nemopteridæ. These insects are very fragile and delicate, somewhat dragon-fly-like in form, but with the most extraordinary hind wings—consisting of a long narrow stalk, with a dark-fiddle-shaped expansion at the end. One species of this family has been found in Chile, while others are known from the warmer parts of the old world. The whole group has become extinct in North America, but the fossil proves that it once existed there. Such a fossil as this not merely throws a flood of light on the past migrations of a peculiar group, but is the first and only indication we have of the past history of its race.
Florissant is famous for its fossil butterflies, having nearly half of the number known in that condition. My wife was particularly anxious to find a fossil butterfly, and often as we went out to work, we asked, would this be the day to yield the coveted treasure? Yet all the first season passed, and no butterfly was obtained. Toward the close of the second season, however, my wife sat down one day at a new place, to see what it might be worth. She had scarcely begun to turn ever the shale when, behold, a truly magnificent specimen! It showed the upper wings, the body and one antenna, the spotting still plainly visible upon the wings. It proved to be an undescribed species, but of a genus still existing in Colorado, though more common southward. When compared with the Scudder collection, it was seen to be the second finest of the butterflies, yielding place only to Scudder's in-