Z PEOCEEDIiSrGS OF THE NATIONAL MUSEUM vol.78 



Anostracan family Opabinidae, the remaining four were distributed 

 among as many families of the Notostraca. These fossils have sub- 

 sequently been considered by Eaymond (1920), Fedotov (1924), and 

 Henriksen (1928). The three authors have come to different con- 

 clusions on some of the forms. On the whole Henriksen's contri- 

 bution is of the most value because he has been able to study new 

 material of certain species, collected by Walcott after the publication 

 of his original paper. Of the Notostraca only one form, Burgessia 

 hella^ is considered as properly placed by either Fedotov or Henrik- 

 sen. Henriksen allows three of Walcott's genera of Anostraca to 

 remain in that group, very properly removing Leanchoilia to the 

 Merostomata. He considers that Tohoia should form the type of a 

 new Anostracan family and that the problematic Bidentia is perhaps 

 allied with Opahinia. Fedotov is very doubtful about Yohoia and 

 Bidentia but all authors are unanimous that Opabinia is rightly 

 placed as an Anostracan. 



The present distribution of the Branchiopoda. — The Branchiopoda 

 rival the Rotifera as the most characteristic invertebrate animals 

 of inland waters. Very few species inhabit the sea, while the group 

 is spread through a very wide range of fresh -water habitats. The 

 three groups of large forms, the Anostraca, Notostraca, and Con- 

 chostraca are to-day essentially crustaceans of temporary seasonal 

 waters, flourishing best in semidesert and steppe environments. The 

 Cladocera, now so widespread in both seasonal and perennial waters, 

 are probably derived from the larger " phyllopods." A very few 

 species of Cladocera inhabit the sea, and these are for the most part 

 neritic. It is clear that chemical conditions have played little part 

 in restricting the Branchiopoda to inland waters. Artemia salina 

 Linnaeus can tolerate a far higher salinity than that of the sea; 

 moreover recent unpublished experiments strongly suggest that cer- 

 tain supposed cases of the limitation of Cladocera by chemical con- 

 ditions are fallacious. The occurrence of an undoubted Anostracan 

 in an ancient marine deposit is therefore of considerable interest to 

 the fresh-water ecologist. Since none of the authors referred to has 

 given entirely satisfactory descriptions of Opabinia it seemed de- 

 sirable to reexamine the material in order to gain additional infor- 

 mation on its structure in the hope that it would throw light on the 

 disappearance of the Branchiopoda from the sea. 



Material examined. — Walcott separated his Opahinia into two 

 species, the typical Opabinia regalis and a doubtful species, 0.? 

 media. The latter, distinguished by its smaller size and lesser num- 

 ber of segments, is based on very poor material in which a segmental 

 count must necessarily be very uncertain. Since no very small speci- 

 mens are included in the material of regalis it is very probable that 



