_Ceraurus pleurexanthemus_ Green 57 Cephalic appendages 58 Thoracic appendages 59 Pygidial appendages 59 Relation of hypostoma to cephalon 59 Restoration of _Ceraurus pleurexanthemus_ 60 The appendages of _Acidaspis trentonensis_ Walcott 61 The appendages of _Cryptolithus_ (see also Part IV) 61 _Cryptolithus tessellatus_ Green 61 Restoration of _Cryptolithus_ 62 Summary on the ventral anatomy of trilobites 64 Comparison of appendages of different genera 64 Coxopodite 64 Cephalon 64 Thorax 66 Pygidium 67 Caudal rami 68 Homology of cephalic appendages with those of other Crustacea 69 Functions of the appendages 70 Antennules 70 Exopodites 70 Endopodites 71 Use of the pygidium in swimming 72 Coxopodites 74 Position of the appendages in life 74 Part II. Structure and habits of trilobites 77 Internal organs and muscles 77 Alimentary canal 77 _Ceraurus pleurexanthemus_ 79 _Calymene senaria_ 80 _Cryptolithus goldfussi_ 80 Summary 81 Gastric glands 82 Summary 84 Heart 85 _Illænus_ 85 _Ceraurus_ and _Calymene_ 85 The median "ocellus" or "dorsal organ" 86 Nervous system 89 Various glands 89 Dermal glands 89 Renal excretory organs 90 Reproductive organs 90 Panderian organs 90 Musculature 91 Flexor muscles 92 Extensor muscles 92 Hypostomial muscles 94