Page:Walcott Cambrian Geology and Paleontology II.djvu/247

 Crustacea and the theoretical views concerning its development. The highly organized merostome Sidneyia inexpectans removed the origin of the Merostomata far back into pre-Cambrian time and seemed to link the problematic Beltina of the Algonkian Belt terrane with the merostomes of Ordovician and Silurian time, and through them with the living Ziphosuridæ. That Branchiopoda of the order Anostraca lived in Cambrian time is not so surprising, but that they should be almost perfectly preserved, and closely allied to the living forms, certainly is unexpected. Opabinia regalis (pl. 27, fig. 6, and pl. 28, fig. 1) is much like Thamnocephalus platyurus Packard, and Burgessia (pl. 27, figs. 1-3) has the dorsal shield and somewhat similar cephalic region of Lepidurus.

Hymenocaris (pl. 31) may be compared with Nebalia, and Carnarvonia (pl. 33) and Tuzoia (pl. 33) with the reticulated carapace of Nebaliopsis typica Sars.

The group of forms represented by Nathorstia (pl. 28, fig. 2), Naraoia (pl. 28, fig. 4), Yohoia (pl. 29, figs. 7-14), and Bidentia (pl. 30, fig. 1) does not appear to have any living representatives.

Viewed as an ancestral fauna of the living Crustacea the Burgess shale fauna foreshadows the Branchiopoda in both its orders, Anostraca and Notostraca; the Ostracoda by the family Indianidæ ; the Malacostraca by the Phyllocarida; and the Merostomata by Aglaspina and Limulava.

The recent Polyartemidæ and Apodidæ are animals that by their remarkable adaptation to conditions are practically immune to agencies that, during geologic time, have destroyed whole races of invertebrate animals. When they became adapted to living in intermittent ponds that depended on rainfall and that might be fresh, brackish, or saline, is unknown. Their wide geographic distribution and the great vitality of their eggs indicate great age, and the discovery of their probable ancestors in such forms as Opabinia (pl. 27, fig. 6, and pl. 28, fig. 1) and Burgessia (pl. 27), in association with a large and