The methods of locomotion may be deduced with some safety from a study of the appendages, and, as has repeatedly been pointed out, all trilobites could probably swim by their use. This swimming was evidently done with the head directed forward, and could probably be accomplished indifferently well with either the dorsal (gastronectic, Dollo) or the ventral (notonectic) side up. If food were sought on the bottom by means of sight, the animal would probably swim dorsal side up, for by canting from side to side it could see the bottom just as easily as though it were ventral side up, and at the same time it would be in position to drop quickly on the prey. In collecting food at the surface, it might swim ventral side up. All trilobites could probably crawl by the use of the appendages, and, as has already been pointed out, there are great differences in the adjustment of the appendages to different methods of crawling. Some crawled on their "toes," some by means of the entire endopodites, and some apparently used the coxopodites to push themselves along. That the normal direction of crawling was forward is indicated by the position of the eyes and sensory antennules. There is no evidence that their mechanism was irreversible, however, and the position of the mouth and the shape of the hypostoma indicate that they usually backed into feeding position. The caudal rami of Neolenus were evidently sensory, and the animal was prepared to go in either direction. The use of the pygidium as a swimming organ, suggested by Spencer (1903, p. 492) on theoretical grounds, developed by Staff and Reck (1911, p. 141) from a mechanical standpoint, and elaborated in the present paper by evidence from the ontogeny, phylogeny, and musculature, provided the animal with a swifter means of locomotion. By a sudden flap of this large fin, a backward darting motion could be obtained, which would be invaluable as a means of escape from enemies. Staff and Reck seem to think that in this movement the two shields were clapped together, and that the animal was projected along with the hinge-like thorax forward. This might be a very plausible explanation in the case of the bivalve-like Agnostidæ, and it is one I had suggested tentatively for that family before I read Staff and Reck's paper. In the case of the large trilobites with more segments, however, it would be more natural to think of a mode of progression in which there was an undulatory movement of the body and the pygidium, up-and-down strokes being produced by alternately contracting the dorsal and ventral muscles. Bending the pygidium down would tend to pull the animal backward, while bringing it back into position would push it forward. It follows, therefore, that one of these movements must have been accomplished very quickly, the other slowly. If the muscle scars have been interpreted properly, the ventral muscles were probably the more powerful, an indication that the animal swam