NO. 6 MIDDLE CAMBRIAN BRANCHIOPODA, ETC. 163 



carapace serve to place Burg,essia in the Branchiopoda under the 

 order Notostraca. 



Among the anostracans Opabinia rcgalis, in its elongate many-seg- 

 mented body, phyllopod-like swimming exopodites and insignificant or 

 rudimentary ambulatory endopodites, small head, and slender body, 

 is very suggestive of an annelidan ancestor. 



These comparisons raise the question as to the relations of the 

 Branchiopoda, Leptostraca (representing the Malacostraca), Trilo- 

 bita, and Merostomata. With the data afforded by the Burgess 

 shale fauna the inter-relationship of the four so-called subclasses is 

 found to be very intimate. 



In Opabinia (pi. 27, fig. 6, and pi. 28, fig. i) and Leanchoilia (pi. 

 31, fig. 6) the typical branchiopod is clearly present. 



In Waptia (pi. 27, figs. 4 and 5) the Leptostraca is very near at 

 hand as developed in Hymenocaris (pi. 31, figs, i and 2). 



In Marrella (pis. 25 and 26) the trilobite is foreshadowed, and 

 Nathorstia (pi. 28, fig. 2) is a generalized trilobite as the trilobite 

 appears to be a specialized branchiopod, adapted largely for creeping 

 on the bottom. The trilobite gives some conception of a possible 

 form between the Branchiopoda and the Aglaspidae of the Merosto- 

 mata. 



Such forms as Habelia (pi. 29, fig. 6), Molaria (pi. 29, figs. 1-5), 

 and Emeraldella (pi. 30, fig. 2) serve to fill in the gap between the 

 Branchiopoda and the Merostomata as represented by Sidneyia ^ and 

 later the Eurypterida. Sidneyia is now known to have a pair of 

 jointed biramous appendages on each of the anterior 9 segments of 

 the body. The inner division or endopodite is a jointed leg adapted 

 for creeping close to the bottom and the outer branch is a lamellated 

 branchial lobe (see Smithsonian Misc. Coll., vol. 57, No. 2, 1911, pi. 

 6, fig. 3, and pi. 7, fig. i ; and text fig. 10 of this paper, p. 206). 



In the following diagram the attempt is made to show the relations 

 of Cambrian crustaceans to a theoretical ancestral stock which for 

 convenience is correlated with the Apodidse. From this stock it is 

 assumed that the Branchiopoda came, and from the Branchiopoda 

 stock three distinct branches were developed prior to or during Cam- 

 brian time. Of these the one of greatest interest in the present con- 

 nection is that on the right of the diagram. In this line of descent 

 it is assumed that the Trilobita are directly descendent from the 

 Branchiopoda and forms grouped under the order Aglaspina derived 

 from the Trilobita. The order Limulava is considered as being inter- 



^ Smithsonian Misc. Coll., Vol. 57, No. 2, 191 1, p. 22. 



