supply of oxygen. Although _Neolenus_ is usually accounted a less primitive form than _Ptychoparia_ or _Triarthrus_, it has much the most primitive type of exopodite yet known. It would appear that the exopodites were originally broad, thin, simple lamellæ, which became broken up, on the posterior side, into fine cylindrical setæ. As development progressed, more and more of the original lamella was broken up until there remained only the anterior margin, which became thickened and strengthened to support the delicate filaments. The setæ in turn became modified from their original simple cylindrical shape to form the wide, thin, blade-like filaments of _Cryptolithus_ and _Ceraurus_. Another possible use of the exopodites is suggested by the action of some of the barnacles, which use similar organs as nets in gathering food and the endopodites as rakes which take off the particles and convey them to the mouth. The exopodites of the trilobite might well set up currents which would direct food into the median groove, where it could be carried forward to the mouth. _Endopodites._ The endopodites were undoubtedly used for crawling; in some trilobites, probably most of them, for swimming; in the case of _Cryptolithus_, and probably others, for burrowing; and probably in all for gathering food, in which function the numerous spines with which they are arrayed doubtless assisted. Various trails have been ascribed to the action of trilobites, and many of them doubtless were made by those animals (see especially Walcott, 1918). Some of these trails seem to indicate that in crawling the animal rested on the greater part of each endopodite, while others, notably the _Protichnites_ recently interpreted by Walcott (1912 B, p. 275, pl. 47), seem to have touched only the spinous tips of the dactylopodites to the substratum. The question of the tracks, trails, and burrows which have been ascribed to trilobites is discussed briefly on a later page; but can not be taken up fully, as it would require another monograph to treat of them satisfactorily.