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Heart of Apus 85 28 Isotelus gigas Dekay. The Panderian organs 91 29 Ceraurus pleurexanthemus Green. Restoration, showing heart, alimentary canal, and extensor muscles 93 30 The same. Longitudinal section of cephalon 95 31 Nileus armadillo Dalman. Moberg's figure of the muscle-scars 95 32 Marrella splendens Walcott. Restoration of the ventral surface 116 33 Triarthrus becki Green. Appendage of the anterior part of the thorax 126 34 Apus. Appendage from the anterior part of the trunk 127 35 Weymouthia nobilis (Ford) 138 36 Naraoia compacta Walcott 145 37 Pagetia clytia Walcott 145 38 Asaphiscus wheeleri Meek 145 39 Pædeumias robsonensis Burling 145 40 Robergia sp. 145 41 Diagram showing possible lines of descent of the Arthropoda 150 42 Triarthrus becki Green. Thoracic appendages 155 43 The same. Pygidial appendages 157 44 The same. Pygidial appendages 158 45 Cryptolithus tessellatus Green. Drawing of the best single specimen 159 46 The same. Part of the thorax and pygidium, with appendages 162

Frontispiece. Charles Emerson Beecher, 1896.

Plates 1-5. Photographs of Triarthrus becki, made by C. E. Beecher.

Plate 6. Photographs of Triarthrus becki (figs. 1-3), Acidaspis trentonensis (fig. 6), and Cryptolithus tessellatus (fig. 7), made by C. E. Beecher. Photographs of the endopodites of a probable species of Calymene (figs. 4, 5)

Plates 7-8. Photographs of Cryptolithus tessellatus, made by C. E. Beecher.

Plate 9. Drawings of Cryptolithus tessellatus, made by C. E. Beecher or under his direction.

Plate 10. Photographs of Isotelus latus and I. maximus, made by C. E. Beecher.

Plate 11. Drawing of a restoration of Ceraurus pleurexanthemus, made by Elvira Wood.

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