NO. 6 MIDDLE CAMBRIAN BRANCHIOPODA, ETC. 1 59 



of the Branchiopoda and Malacostraca is six if we consider the 

 stalked eyes as representing the first pair. 



Eyes = first 

 Antennules = second 

 Antennae = third 

 Mandibles = fourth 

 MaxillulK= fifth 

 Maxillae = sixth 



The stalked eyes are distinctly shown for Opabinia (pi. 28, fig. i), 

 Waptia (pi. 27, fig. 4), and Yohoia (pi. 29, fig. 9), and for Hymeno- 

 caris by specimens not illustrated. Biirgessia (pi. 27) and Marrella 

 (pi. 25, figs. 4 and 5) have sessile eyes and five pairs of cephalic 

 appendages. The sessile eyes, as in the trilobite, probably repre- 

 sent a segment of the cephalic shield. 



The character of the several thoracic appendages is described 

 under each species. So far as determined, the stalked eyes, anten- 

 nules, and antennae are not very unlike those of recent crustaceans 

 of the same orders, and the mandible, maxillula, and maxilla also 

 have the same fundamental structure with modifications to meet the 

 needs of each genus and species. 



The thoracic appendages appear to be based on the typical crusta- 

 cean limb having a protopodite bearing an exopodite and endopodite. 

 There are no recognized modifications of this that would indicate a 

 simpler form. An epipodote (gill) is attached to the protopodite in 

 Marrella (pi. 26, fig. 4), Opabinia (pi. 27, fig. 6), and Malaria 

 (pi. 29, fig. 3). 



The number of thoracic appendages is indicated in the table (p. 158) 

 which gives the number of thoracic segments. Each of these thoracic 

 segments is considered to have had a pair of attached appendages 

 although, as in the case of Apus, the posterior segments may possibly 

 have had more than one pair of appendages. 



Simple, bifid abdominal appendages only appear on the abdominal 

 segments of Anomalocaris (pi. 34, fig. 3). The caudal rami of the 

 abdominal segment vary greatly in form and structure in the crusta- 

 ceans from the Burgess shale. The female of Opabinia appears to 

 have two unsegmented, expanded rami. Waptia (pi. 27, figs. 4 and 

 5) and Yohoia (pi. 29, figs. 8, 11, and 14) have two expanded rami 

 with rudimentary segmentation. Hymenocaris (pi. 31, figs. 3 and 5) 

 and Odaraia (pi. 34, fig. 2) have several cercopods attached to the 

 last abdominal segment, and the trilobite Neolenus (pi. 24, figs, i and 

 la) has two long, slender, jointed rami. 



