NO. 6 



MIDDLE CAMBRIAN BRANCHIOPODA, ETC. 



i6i 



tion in Lipalian time or the period of pre-Cambrian marine sedimen- 

 tation 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 

 Hmited horizon (phyllopod bed) in the Middle Cambrian. The 

 five subclasses are represented as having had a long line of crusta- 

 cean ancestors, a view that if correct would manifestly necessitate a 

 prolonged pre-Cambrian period for the development of the crusta- 

 cean fauna now found in the Burgess shale. As the trilobites are 



Stdneyia'- ■ 



Amiella • 

 Aglaspis 

 Molaria 



Emtraldella'- 



Waptii 



^Hymenocaris 



/ 



/ 



Opabinii 



Theoretical Lines of Descent of Cambrian Crustacea 



probably derived from the same stock as the Branchiopoda, the lines 

 of probable descent of the various genera of the latter in the Bur- 

 gess shale are projected backward into the pre-Cambrian. It may be 

 that some of the genera of the Branchiopoda in the table were de- 

 veloped 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 



^Smithsonian Misc. Coll., Vol. 57, No. i, 1910, p. 14. 

 2 



