In Branchiopoda, the antennæ are variable. In the Notostraca they are vestigial, while in the males of the Anostraca they are powerful and often complexly developed claspers. Either condition might develop from the generalized biramous antennas of Trilobita, but the present evidence indicates a tendency toward obsolescence. Claus' observations indicate that the antennæ of the Anostraca are developments of the exopodites, rather than of the endopodites. The mandibles and maxillæ of the Branchiopoda are greatly reduced, and grouped closely about the mouth. Only the coxopodites of the Trilobita are modified as oral appendages. The trunk limbs of _Apus_ are supposed to be the most primitive among the Branchiopoda, and comparison will be made with them. Each appendage consists of a flattened axial portion, from the inner margin of which spring six endites, and from the outer, two large flat exites (see fig. 34). This limb is not articulated with the ventral membrane, but attached to it, and, if Lankester's interpretation of the origin of schizopodal limbs be correct, then the limb of _Apus_ bears very little relation to that of the Trilobita. In _Apus_ there is no distinct coxopodite and the endobases which so greatly resemble the similar organs in the Trilobita are not really homologous with them, but are developments of the first endite. Beecher's comparison of the posterior thoracic and pygidial limbs of _Triarthrus_ with those of _Apus_ can not be sustained. Neither _Triarthrus_ nor any other trilobite shows any trace of phyllopodan limbs. Beecher figured (1894 B, pl. 7, figs. 3, 4) a series of endopodites from the pygidium of a young _Triarthrus_ beside a series of limbs from a larval _Apus_. Superficially, they are strikingly alike, but while the endopodites of _Triarthrus_ are segmented, the limbs of _Apus_ are not, and the parts which appear to be similar are really not homologous. The similarity of the thoracic limbs in the two groups is therefore a case of parallelism and does not denote relationship. Geologically, the Branchiopoda are as old as the Trilobita, and while they did not have the development in the past that the trilobite had, they were apparently differentiated fully as early. Anostraca, Notostraca and Conchostraca, three of the four orders, are represented in the Cambrian by forms which are, except in their appendages, as highly organized as the existing species. Brief notes on the principal Middle Cambrian Branchiopoda follow: =Burgessia bella= Walcott. Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, p. 177, pl. 27, figs. 1-3; pl. 30, figs. 3, 4. This is the most strikingly like the modern Branchiopoda of any species described by Walcott from the Middle Cambrian, and invites comparison with _Apus_. The carapace is long, loosely attached to the body, and extends over the greater part of the thorax. The eyes are small, sessile, and close to the anterior margin.