Since the earliest times there has been a constant temptation to compare the depressed shields of the trilobites with the similar ones of isopods. Indeed, when _Scrolls_ with its Lichadian body was first discovered about a hundred years ago, it was thought that living trilobites had been found at last. The trilobate body, cephalic shield, sessile eyes, abdominal shield, and pleural extensions make a wonderful parallel. This similarity is, however, somewhat superficial. The appendages are very definitely segregated in groups on the various regions of the body, and while the pleopods are biramous, the thoracic legs are without exopodites (except in very early stages of development of one genus). The Isopoda arose just at the time of the disappearance of the Trilobita, and there seems a possibility of a direct derivation of the one group from the other. It should be pointed out that while the differences of Isopoda from Trilobita are important, they are all of a kind which could have been produced by the development from a trilobite-like stock. For example: Isopoda have a definite number of segments. There is less variation in the number of segments among the later than the earlier trilobites. Isopoda have no facial suture. In at least three genera of trilobites the cheeks become fused to the cranidium and the sutures obliterated. Isopoda have one or two segments of the thorax annexed to the head. While this is not known to occur in trilobites, it is possible that it did. Most Isopoda have a fairly stiff ventral test. The ventral membrane of trilobites would probably have become stiffened by impregnation of lime if the habit of enrollment had been given up. In Isopoda the antennæ are practically uniramous sensory organs. The second cephalic appendages of trilobites are capable of such development through reduction of the exopodite. In the Isopoda the coxopodites are usually fused with the body, remaining as free, movably articulated segments only in a part of the thoracic legs of one suborder, the Asellota. Endobases are entirely absent. This is of course entirely unlike the condition in Trilobita, but a probable modification. In Isopoda there is a distinct grouping of the appendages, with specialization of function. The trilobites show a beginning of tagmata, and such development would be expected if evolution were progressive. In both groups, development from the embryo is direct. Rudiments of exopodites of thoracic legs have been seen in the young of one genus.