Page:Walcott Cambrian Geology and Paleontology II.djvu/246

 between Aglaspina and the Eurypterida, and that the two orders Limulava and Aglaspina serve to connect the Trilobita and the Eurypterida.

From the Eurypterida we pass to the Xiphosura. It is thought that the Phyllocarida, as represented by the group of forms included under the Hymenocarina, came from the Branchiopoda, but on a different line of descent from the Trilobita and the orders grouped under the Merostomata.

The ostracods are assumed to have been derived from the Branchiopoda but on a different line of descent from the Trilobita and Phyllocarida.



I will not attempt further to discuss the various lines of descent of the genera in this preliminary notice, as in the spring of 1913 much more material may be available for study. The outline diagram (p. 161) indicates my present view, though this is tentative pending study and comparison with living forms. Any speculation on the origin of the various invertebrate groups based on the faunas found in the Cambrian must necessarily be very defective, as the pre-Cambrian development extended far back into pre-Cambrian time.

That the Burgess shale crustacean fauna was a tremendous surprise to me and that it will be to all paleozoologists is evident to any one acquainted with what was known of the early Paleozoic