NO. 6 MIDDLE CAMBRIAN BRANCHIOPODA, ETC. I9I 



not to be looked on as a nearly allied or similar group to this or to other 

 tribes. 



Putting out of the question the important difference exhibited in the 

 numerical proportion of the thoracic rings just alluded to, this analogy to the 

 Aspidostraca might certainly have been considered as very close — all the 

 other relations of organization, so far as they can be traced, corresponding 

 very accurately — if it vi^ere not for the structure of the extremities. These, 

 indeed, which are hard, horny, and articulated in a sub-division of the present 

 Aspidostraca, were probably entirely absent in this form in Trilobites ; but in 

 other respects all the typical characters of the two groups will be found to 

 correspond. 



Bernard * concluded that — 



Apus, on account of its richer segmentation, the absence of pleurae on the 

 trunk-segments, and its more membraneous parapodia-like limbs, must be 

 assumed to lie in the direct line upwards from the original annelidian ancestor 

 toward the modern crustacea. The trilobites then must have branched off 

 laterally from this line either once or more than once, in times anterior to the 

 primitive Apus, as forms specialized for creeping under the protection of a 

 hard imbricated carapace. 



In 1895, with the new evidence afforded by the trilobite Triarthrus 

 becki, he concluded " that — 



The trilobites, therefore (as exemplified by Triarthrus), in spite of their 

 extremely primitive mouth-formula, do not stand in the direct line of descent 

 of the Crustacea, but are lateral offshoots, specialized for a creeping manner 

 of life. 



The discovery of caudal rami on Neolenus (pi. 24, figs, i and la) 

 still further accentuates the conclusion of Bernard that the trilobites 

 were derived from the same stock as Apus. This is further strength- 

 ened by the presence in the Middle Cambrian of a form like Nathors- 

 fia transitans (pi. 28, fig. 2), which is essentially a trilobite, but its 

 setiferous thoracic appendages relate it closely to Opabinia regalis 

 (pi. 27, fig. 6). 



Neolenus serratus (Rominger). — A number of specimens of this 

 species show the antennae, jointed thoracic legs, and caudal rami. 

 One of the specimens is illustrated by figure i, plate 24. In this the 

 caudal rami have been displaced and dragged back, bringing a por- 

 tion of the ventral surface of the abdomen. Figure la shows the 

 caudal rami in their normal position. I have already illustrated the 

 filamentous branchial thoracic appendage of this species.^ The re- 

 semblance between these branchiae and the branchiae or gills of the 

 branchiopod Waptia Reldensis (pi. 27, figs. 4 and 5) is very striking. 



^The Systematic Position of the Trilobites, 1894, Pt. i, pp. 429-430. 



*Idem, 189s, Pt. 2, p. 356. 



^ Smithsonian Misc. Coll., Vol. 57, No. 2, 191 1. pi. 6, figs, i and 2. 



