Page:Walcott Cambrian Geology and Paleontology II.djvu/237

 Sub-Class Merostomata

Order Aglaspina, new order

Family Aglaspidæ Clarke

Genus Molaria, new genus

Genus Habelia, new genus

Genus Emeraldella, new genus

Order Limulava Walcott

Family Sidneyidæ Walcott

Genus Sidneyia Walcott

The several genera of the four subclasses (with the exception of the group of malacostracans represented by genera other than Hymenocaris and the trilobitic genera Mollisonia and Tontoia) have approximately the known vertical range in the Cambrian noted in the diagram on page 156.

In addition to representatives of the subclasses Branchiopoda, Malacostraca, Trilobita, and Merostomata, mentioned in this paper, I have added in the table genera of the Merostomata that occur in the Lower Cambrian and Algonkian, respectively, and of the Ostracoda in the Lower Cambrian, in order to present an outline of the lowest known stratigraphic position of the five subclasses of Crustacea. With the exception of the Branchiopoda all of these are known to have representatives in later Paleozoic formations.

The subclass Merostomata is represented by Beltina in the pre-Cambrian; by Amiella in the upper part of the Lower Cambrian, by the latter genus and Habelia, Molaria, Emeraldella, and Sidneyia in the Middle Cambrian Burgess shale; and by Aglaspis and Strabops in the Upper Cambrian.

The Phyllocarida is first known in the Lower Cambrian by Isoxys, a genus that is represented in the Burgess shale. Hymenocaris is well distributed in the lower half of the Middle Cambrian and the order Hymenocarina continues on up into the Ordovician, Silurian, and Devonian.