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Middle-Cambrian invertebrate fossils, showing the diversity of the animal life of that period and the similarity of many of the types to recent forms. The specimens from which the photographs were taken are in the U.S. National Museum. (Illustrations reproduced by permission of the Secretary of the Smithsonian Institution, Washington, D.C.)

FIG. i. Choia carleri Walcott, a silicious sponge.

FIG. 2. Ottoia prolifica Walcott, a gephyrcan annelid.

FIG. 3. Ottoia minor Walcott, another gephyrean annelid.

FIG. 4. Ayslicaia pedunculata Walcott, a TomoplerisAikc annelid. FIG. 5. Canadia spinosa Walcott, a polychaete annelid. FIG. 6. Amiskwia sagitliformis Walcott, a chaetognath. FIG. 7. Waptia fieldensis Walcott, a Mysis-likc crustacean. FIG. 8. Opabinia regalis Walcott, a Branchipus-like crustacean. FIG. 9. Another specimen of Opabinia regalis Walcott. FIG. 10. Burgessia bella Walcott, an Apus-like crustacean. FIG. n. Marrella splendens Walcott, a simple trilobite. FIG. 12. Naraoia compacta Walcott, a curious crustacean.