Although zoologists are not all agreed as to the precise systematic place to be assigned to the Trilobites, there can be little doubt that they were related more or less closely to the most primitive Crustacea, and they are of special interest as preserving for us the stage in which the second pair of appendages were still used as biting jaws, and had not moved forwards in front of the mouth to become antennæ, as in all living Crustacea. Contemporary with some of the earliest Trilobites, however, are undoubted Crustacea, which, so far as we know their structure, are not very different from types now living. In the Cambrian epoch the Branchiopoda appear to be represented by _Protocaris_, which in its general form resembles _Apus_; and there are a variety of genera and species of Ostracoda, although, since their shells alone are preserved, it is not possible to determine their exact relations to existing forms. In the succeeding Ordovician and Silurian epochs we first meet with the remains of Barnacles, and it is interesting to note that some of them are referred to the genera _Pollicipes_ and _Scalpellum_, which are represented by numerous species in the seas of the present day. Along with these, however, are some strange-looking forms (_Turrilepas_, etc.), having the body covered with rows of overlapping plates. If these are really Cirripedes, they must have differed considerably in structure from the more modern types. The Malacostraca are more interesting from the point of view of palæontology than the other subclasses of Crustacea, since the evolution of the group appears to have taken place within the period covered by the fossil records, and it is possible to trace the course of that evolution--at least, in its broad outlines. It has already been pointed out that the most primitive of existing Malacostraca are the Phyllocarida (_Nebalia_ and its allies), which are in several respects intermediate between the higher Malacostraca and the Branchiopoda; and it is interesting to find that fossils apparently belonging to the Phyllocarida are found far earlier than any of the other Malacostraca. In the Cambrian, and more abundantly in the Ordovician and Silurian, there are found Crustacea (Fig. 82) that resemble _Nebalia_ in having a large bivalved carapace, with a movable beak-like plate in front, a projecting abdomen without conspicuous limbs, and a pair of large spines at the sides of the telson. Unfortunately, we have almost no knowledge of the structure of the limbs; but it can hardly be doubted that these very ancient Crustacea were allied to the existing Phyllocarida, and that they included the forerunners of the higher Malacostraca. [Illustration: FIG. 82--_Ceratiocaris papilio_, ONE OF THE FOSSIL PHYLLOCARIDA. (From Lankester's "Treatise on Zoology," after H. Woodward.) _a_, Traces of antennules; _m_, possibly mandibles; _r_, rostral plate]