12 PROCEEDINGS OF THE NATIONAL MUSEUM vol.78 



Paleozoic times. Burgessia hella Walcott (1912) is referred by 

 both Walcott and Henriksen to the Notostraca; my own reexamina- 

 tion of the material adds nothing to their account. This form is of 

 particular interest as its internal organs can be seen in great detail. 

 If a modern Notostracan were similarly fossilised the most promi- 

 nent internal organs visible would undoubtedly be the large maxil- 

 lary glands in the carapace of which no trace can be seen in Burgessia. 

 Their absence in a marine form is in accordance with what is found 

 in other Crustacea, where " excretory " organs are better developed 

 in fresh-water forms than in marine, and function as regulators of 

 water content and osmotic pressure (Schlieper, 1929). 



Systematic Position. — In order to express the considerable differ- 

 ences existing between modern species of Anostraca and Opahinia 

 and its ally Rochdalia^ the order may be conveniently divided into 

 two suborders in the following way : 



Suborder 1. EUANOSTRACA 



Anostraca with 19 or more segments of which at least 11 are 

 pedigerous, followed by a postpedigerous, postgenital, region of at 

 least 8 segments. Trunk appendages with branchise and flabellum. 

 Caudal furca present (except in Thamnocephalus) median frontal 

 process, if developed, strongly bifurcate. 



Family ^ 1, Polyartemidae Simon, Recent, Circumarctic. 



Family 2, Artemidae Grochowski, Eecent and Eocene, Cosmo- 

 politan. 



Family 3, Branchipodidae Daday, Recent, Old World. 



Family 4, Chirocephalidae Daday, Recent, Old World and North 

 America. 



Family 5, Streptocephalidae Daday, Recent, Old World and 

 North America. 



Suborder 2. PALAEANOSTRACA 



Anostraca with not more than 17 segments, of which 11-15 are 

 pedigerous followed by a very reduced postpedigerous region. 

 Trunk appendages apparently deficient in exites. Caudal furca 

 absent in known forms. 



Family 1, Opabinidae Walcott. 



Fifteen pedigerous segments followed by a postpedigerous portion 

 divided into two parts, of which at least the anterior one is i:ire- 

 sumably segmental. Frontal process bifurcated only at the apex. 



Type Opabinia Walcott with one certain species, O. regalis Wal- 

 cott, Middle Cambrian, British Columbia. 



= Daday (1910) and Barnard (1929). 



