In both there are five pairs of cephalic and eight of thoracic appendages, while those of the abdomen of Hymenocaris are not known. In both, the antennules are less developed than the antennæ. In the Nebaliidæ the antennules show evidence of having been originally double (they are obviously so in the embryo), while they are single in _Hymcnocaris_. In both, the antennæ are simple. The remaining cephalic organs are too little shown by the specimen from the Middle Cambrian to allow detailed comparison. The mandibles, maxillulæ, and maxillæ of _Nebalia_ are, however, of types which could be derived from the trilobite. In three of the genera of the Nebaliidæ, the eight pairs of thoracic limbs are all similar to one another, though those of the genera differ. All are biramous. The limbs of _Hymcnocaris_ can apparently be most closely correlated with those of _Nebalia antarctica_, in which the endopodite consists of short flattened segments, and the exopodite is a long setiferous plate. Epipodites are present in both _Nebalia_ and _Hymcnocaris_. So far as the appendages of _Hymenocaris_ are known, they agree very well with those of the Nebaliidæ, and since they are of the trilobite type, it may safely be stated that the Trilobita and Malacostraca are closely related. _2. Syncarida._ Walcott (1918, p. 170) has compared the limbs of _Neolenus_ with those of the syncarid genera _Anaspides_ and _Koonunga_. These are primitive Malacostraca without a carapace, but as they have a compressed test and _Anaspides_ has stalked eyes, their gross anatomy does not suggest the trilobite. The thoracic appendages are very trilobite-like, since the endopodite has six segments (in _Anaspides_) and a multisegmented setiferous exopodite. The coxopodites, except of the first thoracic segment, do not, however, show endobases, and those which are present are peculiar articulated ones. The cephalic appendages are specialized, and the antennules double as in most of the Malacostraca. External epipodites are very numerous on the anterior limbs. This group extends back as far as the Pennsylvanian and had then probably already become adapted to fresh-water life. It may be significant that the Palæozoic syncarids appear to have lacked epipodites. While differing very considerably from the Trilobita, the Syncarida could have been derived from them. _3. Isopoda._