derived primitive Malacostraca retaining biramous antennæ, but with a carapace and reduced pleural lobes and pygidium. From this offshoot were probably derived the Ostracoda, the Cirripedia, and the various orders of the Malacostraca, with the possible exception of the Isopoda. I have suggested independent origins of the Copepoda and Isopoda, but realize the weighty arguments which can be adduced against such an interpretation. [Illustration: Fig. 36.--_Naraoia compacta_ Walcott. An outline of the test, after Walcott. Natural size.] [Illustration: Fig. 37.--_Pagetia clytia_ Walcott. An eodiscid with compound eyes. After Walcott. × 5.] [Illustration: Fig. 38.--_Asaphiscus wheeleri_ Meek. A representative trilobite of the Middle Cambrian of the Pacific province. After Meek. × 1/2.] [Illustration: Fig. 39.--_Pædeumias robsonensis_ Burling. Restored from a photograph published by Burling. × 1/4.] [Illustration: Fig. 40.--_Robergia_ sp. Restored from fragments found in the Athens shale (Lower Middle Ordovician), at Saltville, Va. Natural size.] It is customary to speak of the Crustacea and Trilobita as having had a common ancestry, rather than the former being in direct line of descent from the latter, but when it can be shown that the higher Crustacea are all derivable from the Trilobita, and that they possess no characteristics which need have been inherited from any other source than that group, it seems needless to postulate the evolution of the same organs along two lines of development. I can not go into the question of which are more primitive, sessile or stalked eyes, but considering the various types found among the trilobites, one can but feel that the stalked eyes are not the most simple. While no trilobite had movable stalked eyes, it is possible to homologize free cheeks with such structures. They always bear the visual surface, and, in certain trilobites (_Cyclopyge_), the entire cheek is broken up into lenses. Since a free cheek is a separate entity, it is conceivable that it might lie modified into a movable organ. EVOLUTION OF THE MEROSTOMATA. It has been pointed out above that the Limulava (_Sidneyia_, _Amiella_, _Emeraldella_) have certain characteristics in common with the trilobites on the one hand and the Eurypterida on the other. These relationships have been emphasized by Walcott, who derives the Eurypterida through the Limulava and the Aglaspina from the Trilobita. The Limulava may be derived from the Trilobita, but indicate a line somewhat different from that of the remainder of the Crustacea. In this line the second cephalic appendages do not become antennæ and the axial lobe seems to broaden out, so that the pleural lobes become an integral part of the body. As in the modern Crustacea, the pygidium is reduced to the anal plate, and this grows out into a spine-like telson.