Part I. the Appendages of Trilobites 20

Terminology 20

The appendages of Neolenus 21 Historical 21 Neolenus serratus (Rominger) 21 Cephalon 21 Thorax 22 Pygidium 23 Epipodites and exites 23 Description of individual specimens 23 Restoration of Neolenus 30 Nathorstia transitans Walcott 31

The appendages of Isotelus 32 Historical 32 Isotelus latus Raymond 34 Isotelus maximus Locke 35 Restoration of Isotelus 37 Isotelus gigas Dekay 37 Isotelus arenicola Raymond 39

The appendages of Triarthrus (see also Part IV) 39 Triarthrus becki Green 39 Historical 40 Restoration of Triarthrus 42 Relation of cephalic appendages to marking on dorsal surface of glabella 43 Anal plate 44

The appendages of Ptychoparia 45 Ptychoparia striata (Emmrich) 45 Ptychoparia cordilleræ (Rominger) 45 Ptychoparia permulta Walcott 45

The appendages of Kootenia 46 Kootenia dawsoni Walcott 46

The appendages of Calymene and Ceraurus 46 Historical 46 Comparison of the appendages of Calymene and Ceraurus with those of Triarthrus 47 Spiral branchiæ 48 Ventral membrane 50 Appendifers 51

Calymene senaria Conrad 52 Cephalic appendages 52 Thoracic appendages 53 Pygidial appendages 54 Relation of hypostoma to cephalon in Calymene 55 Restoration of Calymene 56

Calymene sp. ind. 56

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