Page:Walcott Cambrian Geology and Paleontology II.djvu/243

 in Lipalian time or the period of pre-Cambrian marine sedimentation of which no known part is present on the existing continents.

The known stratigraphic position of the various genera is shown by the diagram on page 156. In this 13 genera are found only at one limited horizon (phyllopod bed) in the Middle Cambrian. The five subclasses are represented as having had a long line of crustacean ancestors, a view that if correct would manifestly necessitate a prolonged pre-Cambrian period for the development of the crustacean fauna now found in the Burgess shale. As the trilobites are



probably derived from the same stock as the Branchiopoda, the lines of probable descent of the various genera of the latter in the Burgess shale are projected backward into the pre-Cambrian. It may be that some of the genera of the Branchiopoda in the table were developed in early Cambrian time, but of this we have no evidence.

A suggested scheme of descent of the genera in the table and other Cambrian genera, with the exception of the genera of the Trilobita, is shown in the above diagram.

The Lower Cambrian formations have only been searched in a