Page:Walcott Cambrian Geology and Paleontology II.djvu/273

 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 were 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 pleuræ 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 (pl. 24, figs. 1 and 1a) still further accentuates the conclusion of Bernard that the trilobites were derived from the same stock as Apus. This is further strengthened by the presence in the Middle Cambrian of a form like Nathorstia transitans (pl. 28, fig. 2), which is essentially a trilobite, but its setiferous thoracic appendages relate it closely to Opabinia regalis (pl. 27, fig. 6).

Neolenus serratus (Rominger).— A number of specimens of this species show the antennæ, jointed thoracic legs, and caudal rami. One of the specimens is illustrated by figure 1, plate 24. In this the caudal rami have been displaced and dragged back, bringing a portion of the ventral surface of the abdomen. Figure 1a shows the caudal rami in their normal position. I have already illustrated the filamentous branchial thoracic appendage of this species. The resemblance between these branchiæ and the branchiæ or gills of the branchiopod Waptia fieldensis (pl. 27, figs. 4 and 5) is very striking.