This prophecy was fulfilled by the discovery of the specimens which Walcott described as _Pædeumias transitans_, a species which is said by its author to be a "form otherwise identical with _O. thompsoni_, [but] has rudimentary thoracic segments and a _Holmia_-like pygidium posterior to the fifteenth spine-bearing segment of the thorax." A good specimen of this form was found at Georgia, Vermont, associated with the ordinary specimens of _Olenellus thompsoni_, and I believe that it is merely a complete specimen of that species. _Olenellus gilberti_, which was formerly supposed to have a limuloid telson, has now been shown by Walcott (Smithson. Misc. Coll., vol. 64, 1916, p. 406, pl. 45, fig. 3) to be a _Mesonacis_ and to have seven or eight thoracic segments and a small plate-like pygidium back of the spine-bearing fifteenth segment. All indications are that the spine was not in any sense a pygidium. Walcott states that _Olenellus_ resulted from the resorption of the rudimentary segments of forms such as _Mesonacis_ and _Pædeumias_, leaving the spine to function as a pygidium. This would mean the cutting off of the anus and the posterior part of the alimentary canal, and developing a new anal opening on the spine of one of the thoracic segments! If the spine of the fifteenth segment is not a pygidium, could it be used, as Dollo postulates, as a pushing organ? Presumably not, for though in entire specimens of _Olenellus_ (_Pædeumias_) it extends back beyond the pygidium, it probably was borne erect, like the similar spines in _Elliptocephala_, and not in the horizontal plane in which it is found in crushed specimens. While this removes some of the force of Dollo's argument, his conclusion that _Olenellus_ was a crawling, burrowing animal living in well lighted shallow waters was very likely correct. The long, annelid-like body indicates numerous crawling legs, there is no swimming pygidium, and the fusion of the cheeks in the head makes a stiff cephalon well adapted for burrowing. Staff and Reck have pointed out that _Dalmanites limulurus_ was not entirely a crawler, but, as shown by the large pygidium, a swimmer as well. This kind of trilobite probably represents the normal development of the group in Ordovician and later times. The Phacopidæ, Proëtidæ, Calymenidæ, and other trilobites of their structure could probably crawl or swim equally well, and could escape enemies by darting away or by "digging themselves in."