ART. 4 CAMBRIAN CONCHOSTRACA — ULRICH AND BASSLER 6 



the shell is different, being thinner, more membranaceous, often 

 flexible, and, if not strictly corneous, at least less calcareous than in 

 the Ostracoda. Again, the valves are not so completely separated, 

 there being, indeed, some reason for believing that the valves were 

 always tightly joined along the back, often perhaps by fusion of the 

 cardinal edges. In the Ostracoda the valves are always completely 

 separated, the hingement along the back being efTected by mere over- 

 laps or by means of interlocking bars or teeth. In the Bradoriidae, 

 finally, the margins of the valves, except along the back, are apart, 

 the edges being so opposed that a narrow slit separates them. A 

 somewhat similar condition occurs among Ostracoda only in the Cyp- 

 ridinidae in which the closed valves leave an incision and often a. 

 keyhole-like slit beneath the anterior "hook." 



For the reasons mentioned we are confident that the Bradoriidae 

 and allied Cambrian Crustacea are not true Ostracoda. However, as 

 they precede the Ostracoda in time, it is quite probable that the latter 

 were derived from the earlier Bradoriidae and Beyrichonidae. That 

 they stand in such relationship is indicated by the fact that the Leper- 

 ditiidae, the oldest representatives of the true Ostracoda, present 

 more resemblances to these Cambrian bivalves than can be seen, with 

 the exception possibly of the Cypridinidae, in any of the succeeding 

 genera of the order. The Cypridinidae in that case may represent 

 a reversion to ancestral characteristics — a condition indicated in 

 Carboniferous representatives of other classes of organisms. 



The correct classification of the small Cambrian bivalved Crustacea 

 is a difficult question. If the superorder Branchiopoda were to 

 include the Phyllocarida as an order or were considered as closel}^ 

 allied, we would not hesitate to place them in the same general associ- 

 ation, as we are impressed with the view that these Cambrian forms 

 are an early phase or offshoot of the stock from which the Branchio- 

 poda and Phyllocarida on the one hand and the Ostracoda on the 

 other were derived. 



To-day the term Branchiopoda is emploj^ed as a superorder tc 

 include the three distinct orders Anostraca, Notostraca, and Concho 

 straca, formerly considered as groups of the old division Phyllopoda^ 

 and a fourth order Cladocera, while the superorder Malacostraca 

 contains all the higher Crustacea with the Phyllocarida as its first 

 division. The dorsal nodes and ridges marking the valves of the 

 Bradoriidae and Beyrichonidae suggest closer relationship with the 

 usually much larger Phyllocarida than with the typical fossil Phyllo- 

 poda (as Estheria and Leaia) of the order Conchostraca whose average 

 size is about as in their supposed Cambrian progenitors. Perhaps 

 this supposed relationship is only a case of parallel development, but 

 at any rate it is obvious that until the body segments and appendages 

 of these particular families have been discovered their exact classifica- 



