NO. 6 MIDDLE CAMBRIAN BRANCHIOPODA, ETC. 1 5/ 



The oldest known representative of the Ostracoda is Indiana 

 dermatoidea from the Lower Cambrian/ Several genera of ostra- 

 cods range up through the Middle and Upper Cambrian. A fine, 

 small species occurs in the Burgess shale. 



The Trilobita begins with Nevadia deep down in the Lower Cam- 

 brian ^ and predominates in all later Cambrian faunas. 



I do not know of any genera of the Branchiopoda in the 

 Cambrian other than those described in this paper from the Burgess 

 shale and the single specimen of Protocaris from the upper part of 

 the Lower Cambrian.'' 



That a large and varied crustacean fauna preceded and followed 

 that of the Burgess shale is certain, and large additions to our infor- 

 mation of it will undoubtedly be forthcoming in the near future. 



STRUCTURAL FEATURES 

 Exoskeleton. — Among the Anostraca there is no true shell, the 

 external cuticle being little more than a membrane that is thicker in 

 the cephalic region and on the telson, if the latter is present. Among 

 the notostracans the carapace varies from the simple form seen in 

 Burgessia (pi. 27) to the double shield of Naraoia (pi. 28). The 

 malacostracans all have a strong bivalve carapace, as shown on plates 



31-34- 



The carapace of Marrella (pi. 25, fig. i) is most interesting. The 

 eyes on the anterior margin, the large antennules ( ?), and the great 

 posterior dorsal spines indicate a great modification of and advance 

 over the simple primitive shield resulting from a fold of the cuticle of 

 the fifth segment of the head. The shield of Burgessia (pi. 27, figs. 

 1-3) is simple, and that of Naraoia (pi. 28, fig. 4) simple over the 

 head and more complex over the thorax. 



In Waptia (pi. 27, figs. 4 and 5) the shield has passed nearly to the 

 bivalve stage of the Hymenocarina. It appears to be a transition 

 between the simple bent shield of Burgessia and the bivalve carapace 

 of Hymenocaris (pi. 31). 



The bivalve carapaces of Tusoia and Carnarvonia are so similar 

 to the carapace of living forms of the Nebaliacea that there is little 

 question of the intimate relationship between them. The reticulated 

 surface on the large carapaces of Carnarvonia (pi. 33, fig. i) and 



^ Tenth Ann. Rept. U. S. Geol. Survey, 1891, p. 626, pi. 80, figs, i and la. 

 TTie genus Indiana Matthew is described in the Canadian Record of Science, 

 Vol. 8, 1902, p. 460. 



^Smithsonian Misc. Coll., Vol. 53, No. 6, 1911, pp. 249 and 258. 



'Tenth Ann. Rept. U. S. Geol. Survey, 1891, p. 629, pi. 81, fig. 6. 



