NO. 3 BURGESS SHALE FOSSILS WALCOTT 37 



6. The coxopodite did not serve as a gnathobase. 



I and 2 are considered to be more primitive characters. 

 3, 4, 5, and 6 less primitive. 



My present conckision is that Marrella is a less primitive form than 

 the Apodidae, and while a more primitive form than the trilobite it is 

 nearer the latter than the Apodidae, and should be grouped near it 

 but not with the Trilobita. At the time of my preliminary examination 

 of the crustaceans then known to me from the Burgess shale I placed 

 Marrella and Nathorstia as progenitors of the trilobite/ but with our 

 present information Marrella will be placed with Burgessia, Nathorstia 

 being left under Trilobita. 



COMPARISON WITH CRUST.\CEANS 



Marrella and the BraneJiiopoda. — Marrella, with its sessile eyes, 

 carapace-like cephalic shield, labrum attached to the doublure, numer- 

 ous trunk limbs, and the large mandible, suggests the Apodidae, but 

 when we consider the well-developed antennae, large removable spine 

 attached to the cephalic shield, biramous trunk limbs on each body 

 segment consisting of a fully developed endopodite and exopodite, 

 and the absence of caudal rami, the conclusion is that Marrella repre- 

 sents a more advanced stage in the evolution of the Crustacea than 

 Apiis and its allies. The biramous limb of Marrella, like that of the 

 trilobite, undoubtedly passed through the foliaceous or multiramous 

 limb stage in its evolution, probably in pre-Cambrian time. 



Marrella differs from the Branchiopoda in : 



a. Absence of lobcd multiramous foliaceous trunk limbs with gnatho- 



bases and in the presence of biramous trunk limbs with protop- 

 odite, jointed endopodite (leg), and jointed exopodite. 



b. Absence of f ureal rami. 



c. Presence of a pair of biramous limbs on each trunk segment back 



to the telson. 



Marrella includes the following characters of the Brancliiopoda: 



a. A true carapace arising from a fold of the integument. 



b. A labrum attached to the reflected margin or " doublure " of the 



carapace. 



c. A large mandible serving as a jaw in the process of mastication. 



Plesiotypes.—U. S. N. M., Nos. 83486a-i. 

 '.Smithsonian Misc. Coll., vol. 57, p. 161, 1912. 



