Page:Smithsonian Miscellaneous Collections, Volume 85.djvu/127

Rh # The coxopodite did not serve as a gnathobase.
 * 1 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.

Marrella and the Branchiopoda.—Marrella, with its sessile eyes, carapace-like cephalic shield, labrum attached to the doublure, numerous 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 represents a more advanced stage in the evolution of the Crustacea than Apus 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:
 * 1) Absence of lobed multiramous foliaceous trunk limbs with gnathobases and in the presence of biramous trunk limbs with protopodite, jointed endopodite (leg), and jointed exopodite.
 * 2) Absence of fureal rami.
 * 3) Presence of a pair of biramous limbs on each trunk segment back to the telson.

Marrella includes the following characters of the Branchiopoda:
 * 1) A true carapace arising from a fold of the integument.
 * 2) A labrum attached to the reflected margin or "doublure" of the carapace.
 * 3) A large mandible serving as a jaw in the process of mastication.

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