24. _Ceraurus pleurexanthemus_ Green. Restoration of a longitudinal section 81 25. _Cryptolithus tessellatus_ Green. Cheek showing the genal cæca 84 26. _Illænus._ Volborth's figure of the heart 85 27. 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. HISTORICAL REVIEW.