Page:The Scientific Monthly vol. 3.djvu/328

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��THE SCIENTIFIC MONTHLY

��transparent integument. Walcotf s researches on this snperb series have brought out two important points: first, the great antiquity of the chief invertebrate groups and their high degree of specialization in Early Cambrian times, which makes it necessary to look for their origin far back in the pre-Cambrian ages ; and, second, the extraordinary per-

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��Fig. 4. Thb Wobu) in Middle Cambbian. (Acadian ob Pabadoxidbs) Tiue.

After Schuchert.

��sistence of type among members of all the inyertebrate phyla from the Mid-Cambrian to the present time, so that sea-forms with an antiquity of 25 million years can be placed side by side with existing sea-forms with very obvious similarities of function and structure as in the series arranged for these lectures by Mr. Roy W. Miner, of the American Museum of Natural History.

Except for the trilobites, the existence of Crustacea in Cambrian times was unknown until the discovery of the primitive shrimp-like form, Burgessia bella (Fig. 5), a true crustacean, which may be com- pared with Apus lucasanus, a member of the most nearly allied recent group. We observe a close correspondence in the shape of the chitinous shield (carapace), in the arrangement of the leaf -like locomotor ap- pendages at the base of the tail, and in the clear iotemal impressions in Burgessia of the so-called "kidneys" with their branched tubules. The position of these organs in Apus is indicated by the two light areas on the carapace. Other specimens of Burgessia found by Walcott show that the tapering abdominal region and tail are jointed as in Apus,

The age of the armored merostome arthropods is also thrust back to Mid-Cambrian times by the discovery of several genera of Aglas-

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