Even in Middle Cambrian times some trilobites (e. g., _Paradoxides_) seem to have enlarged the capacity of the stomach and taken vegetable matter, but later, in the Upper Cambrian and Ordovician, when the development of cephalopods and fishes caused great competition for all animal food, dead or alive, most trilobites seem to have become omnivorous. This is of course shown by the swollen glabella, with reduced lateral furrows, and, in the case of a few species (_Calymene_, _Ceraurus_), the known enlargement of the stomach. _Cryptolithus_ is the only trilobite which has furnished any actual evidence as to its food. From the fact that the alimentary tract is found stuffed from end to end with fine mud, and because it is known to have been a burrower, it has been suggested by several that it was a mud feeder, passing the mud through the digestive tract for the sake of what organic matter it contained. Spencer (1903, p. 491) has suggested a modification of this view: The phyllopods appear to feed by turning over whilst swimming and seizing with their more posterior appendages a little mud which swarms with infusoria, etc. This mud is then pushed along the ventral groove to the mouth. Casts, of the intestine of trilobites are still found filled with the mud. _Ceraurus_ and _Calymene_ also must have occasionally swallowed mud in quantity, otherwise the form of the alimentary canal could not have been preserved as it is in a few of Doctor Walcott's specimens. TRACKS AND TRAILS OF TRILOBITES.