NO. 6 MIDDLE CAMBRIAN BRANCHIOPODA, ETC. 165 



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 Ziphosuridse. 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 (pi. 27, fig. 6, and 

 pi. 28, fig. i) is much like Thamnocephalus platyurus Packard,* 

 and Burgessia (pi. 27, figs. 1-3) has the dorsal shield and somewhat 

 similar cephalic region of Lepidurus. 



Hymenocaris (pi. 31) may be compared with Nebalia, and Car- 

 narvonia (pi. 33) and Tusoia (pi. 33) with the reticulated carapace 

 of Nebaliopsis typica Sars.^ 



The group of forms represented by Nathorstia (pi. 28, fig. 2), 

 Naraoia (pi. 28, fig. 4), Yohoia (pi. 29, figs. 7-14), and Bidentia 

 (pi. 30, fig. i) 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, Anos- 

 traca and Notostraca ; the Ostracoda by the family Indianidge ' ; the 

 Malacostraca by the Phyllocarida ; and the Merostomata by Agla- 

 spina and Limulava.° 



SURVIVAL OF THE BRANCHIOPODA 

 The recent Polyartemidse and Apodidse are animals that by their 

 remarkable adaptation to conditions are practically immune to agen- 

 cies that, during geologic time, have destroyed whole races of inverte- 

 brate 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 (pi. 2y, fig. 6, and pL 

 28, fig. i) and Burgessia (pi. 27), in association with a large and 



^ Smithsonian Misc. Coll., Vol. 57, No. 2, 191 1, pp. 19-28, pis. 2-7. 



^ U. S. Geog. and Geol. Surv. Territories, 12th Ann. Rept., Pt. i, 1883, pp. 

 353-355- Text-fig. 23. 



^ Challenger Rept. 1887, Vol. 19, Pt. 56, pi. 3, figs, i, 5, and 6. 



* Name proposed in MSS. by Ulrich and Bassler. 



■^This was referred to as a subfamily of the Eurypterida in 191 1, but its 

 characters are such that it now seems desirable to consider its typical genus, 

 as representing a family of the Merostomata. 



