Page:Walcott Cambrian Geology and Paleontology II.djvu/239

 The oldest known representative of the Ostracoda is Indiana dermatoidea from the Lower Cambrian. Several genera of ostracods range up through the Middle and Upper Cambrian. A fine, small species occurs in the Burgess shale.

The Trilobita begins with Nevadia deep down in the Lower Cambrian and predominates in all later Cambrian faunas.

I do not know of any genera of the Branchiopoda in the Cambrian other than those described in this paper from the Burgess shale and the single specimen of Protocaris from the upper part of the Lower Cambrian.

That a large and varied crustacean fauna preceded and followed that of the Burgess shale is certain, and large additions to our information of it will undoubtedly be forthcoming in the near future.

Exoskeleton.—Among the Anostraca there is no true shell, the external cuticle being little more than a membrane that is thicker in the cephalic region and on the telson, if the latter is present. Among the notostracans the carapace varies from the simple form seen in Burgessia (pl. 27) to the double shield of Naraoia (pl. 28). The malacostracans all have a strong bivalve carapace, as shown on plates 31-34. The carapace of Marrella (pl. 25, fig. 1) is most interesting. The eyes on the anterior margin, the large antennules (?), and the great posterior dorsal spines indicate a great modification of and advance over the simple primitive shield resulting from a fold of the cuticle of the fifth segment of the head. The shield of Burgessia (pl. 27, figs. 1-3) is simple, and that of Naraoia (pl. 28, fig. 4) simple over the head and more complex over the thorax.

In Waptia (pl. 27, figs. 4 and 5) the shield has passed nearly to the bivalve stage of the Hymenocarina. It appears to be a transition between the simple bent shield of Burgessia and the bivalve carapace of Hymenocaris (pl. 31).

The bivalve carapaces of Tusoia and Carnarvonia are so similar to the carapace of living forms of the Nebaliacea that there is little question of the intimate relationship between them. The reticulated surface on the large carapaces of Carnarvonia (pl. 33, fig. 1) and