Male 555 Order II.--Abdominalia 563 Cryptophialus minutus 566 Female 566 Male 584 Order III.--Apoda 587 Proteolepas bivincta 589 Synopsis et Index Systematicus 606 Synopsis et Index Systematicus Specierum, et recentium, et fossilum 611 Description of plates 641 Plate 1. Balanus tintinnabulum 641 Plate 2. Genus Balanus 641 Plate 3. Genus Balanus 642 Plate 4. Genus Balanus 642 Plate 5. Genus Balanus 642 Plate 6. Genus Balanus 643 Plate 7. Genus Balanus 643 Plate 8. Genus Balanus 644 Plate 9. Sub-Genus Acasta 644 Plate 10. Genus Tetraclita 645 Plate 11. Genera Tetraclita and Elminius 645 Plate 12. Genera Elminius and Pyrgoma 646 Plate 13. Genera Pyrgoma and Creusia 646 Plate 14. Genera Creusia and Chelonobia 647 Plate 15. Genera Chelonobia and Coronula 648 Plate 16. Genus Coronula 649 Plate 17. Genera Platylepas, Tubicinella, and Xenobalanus 651 Plate 18. Genus Chthamalus 652 Plate 19. Genera Chthamalus, Chamæsipho, and Pachylasma 652 Plate 20. Genera Pachylasma, Octomeris, and Catophragmus 653 Plate 21. Genus Verruca 654 Plate 22. Alcippe lampas 655 Plate 23. Genera Alcippe and Cryptophialus 658 Plate 24. Genera Cryptophialus and Proteolepas 660 Plate 25. Genera Proteolepas and Balanus 662 Plate 26. Structure of the Mouth and Thorax 664 Plate 27. Nervous System and Senses 666 Plate 28. Cementing Apparatus 667 Plate 29. Cirri and Larvæ, first stages 669 Plate 30. Larvæ of Lepas: second and last stages of development 671