Fig. 8.—Neolenus serratus (Rominger). A restoration of the ventral surface, with the endopodites omitted from one side, to permit a better exposition of the exopodites. The position and number of the appendages about the mouth are in considerable doubt. Restored by Doctor Elvira Wood under the supervision of the writer. About one-half larger than the average specimen.
Nathorstia transitans Walcott.
Illustrated: Walcott, Smithson. Misc. Coll., vol. 57, 1912, pl. 28, fig. 2.
The badly preserved specimen on which this genus and species was based is undoubtedly a trilobite, but for some reason it does not find a place in Walcott's recent article on "Appendages" (1918). The preservation is different from that of the associated trilobites, being merely a shadowy impression, indicating a very soft test. The general outline of the body, the position of the eye, and even a trace of spines about the pygidium (in the figure) are similar to those of Neolenus, and I would venture the suggestion that Nathorstia transitans is a recently moulted Neolenus serratus, still in the "soft-shelled" condition. Even if not a Neolenus, it is probable, from the state of preservation, that it is an animal which had recently cast its shell.
Walcott describes such fragments of appendages as remain, as follows:
Head. A portion of what may be an antenna projects from beneath the right anterior margin; from near the left posterolateral angle a large four-jointed appendage extends backward. I assume that this may be the outer portion of the large posterior appendage (maxilla) of the head.
Thorax. Traces of several slender-jointed thoracic legs project from beneath the anterior segments and back of these on the right side more or less of six legs have been pushed out from beneath the dorsal shield; these are composed of three or four long slender joints; fragments of the three proximal joints indicate that they are shorter and larger and that they have a fringe of fine setæ. Indications of a branchial lobe (gill) are seen in two specimens where the legs are not preserved. This is often the case both among the Merostomata (pl. 29, fig. 3, Molaria) and Trilobita (pl. 24, fig. 2, Ptychoparia).
Two caudal rami project a little distance beneath the posterior margin of the dorsal shield.
This latter feature of course suggests Neolenus. The other appendages are too poorly preserved to allow comparison without seeing the specimen.
The specific name was given "on account of its suggesting a transition between a Merostome-like form, such as Molaria spinifera, and the trilobites." In what respect it is transitional does not appear.