Considering, first, the position of the appendages in crawling, it appears that different trilobites used their appendages in different ways. _Neolenus_ had compact stocky legs, which allowed little play of one segment on another, as is shown by the wide joints at right angles to the axis of the segment. Such limbs were stiff enough to support the body when the animal was crawling beneath the water, where of course it weighed but little. That such a crawling attitude was adopted by trilobites has been shown by Walcott in his explanation of the trails known as _Protichnites_ (1912 B, p. 278). Many trilobites probably crawled in this way, on the tips of the toes, so to speak. In such the limbs would probably extend downward and outward, with the flattened sides vertical. The limb of _Triarthrus_, however, is of another type. The endopodites are long, slender, flexibly jointed, the whole endopodite probably too flexible to be used as a unit as a leg must be in walking on the "toes." The proximal segments of the thoracic and pygidial endopodites are, however, triangular instead of straight-sided, and, the spine-bearing apex of the triangle being ventral, it enabled the endopodites to get a grip on the bottom and thus push the animal forward. This method of progression was more clumsy and less rapid than that of Neolenus, but it sufficed. The natural position of the endopodite when used in this way would seem to be with the flattened sides of the segments standing at an angle of 30 to 45 with the vertical, thus allowing a good purchase on the bottom and at the same time offering the minimum resistance to the water when moving the appendages forward. _Isotelus_ has endopodites different from those of either _Neolenus_ or _Triarthrus_. They are composed of cylindrical segments, the joints indicating a certain amount of flexibility. Since there is no method by which the segments may get a purchase on the bottom other than by pushing with the distal ends, it would seem at first thought that _Isotelus_, like Neolenus, crawled on its "toes." The endopodites of _Isotelus_ are however, short and feeble when compared with the size of the test, while the endobases of the coxopodites are extraordinarily developed. These facts, together with certain trails, strongly suggest the use of the coxopodites as the primary ambulatory organs, the endopodites probably assisting. In this event, the position of the endopodites and coxopodites would be downward, both outward and inward from the point of attachment, and the motion both backward and forward. The fact that in the specimens as preserved the coxopodites point backward and the endopodites forward indicates that the limb as a whole swung on a pivot at the appendifer. It is of course natural to suggest that the coxopodites and endopodites of all the trilobites with wide axial lobes, _Nileus_, _Bumastus_, _Homalonotus_, etc., were developed in this same way.