[Illustration: Fig. 7.--A restored section across the thorax of _Neolenus serratus_, showing the probable form of attachment of the appendages, their relation to the ventral membrane, and the jaw-like endobases of the coxopodites.] _Measurements:_ The cephalon is 24 mm. long and about 60 mm. wide. The coxopodite of the third appendage is about 10 mm. long and 5.5 mm. wide at the widest point. The corresponding endopodite is 19 mm. long and projects 11 mm. beyond the margin, which is about 5 mm. further than it would project were the appendage restored to its proper position. RESTORATION OF NEOLENUS. (Text figs. 7, 8.) This restoration is based upon the information obtained from the studies which have been detailed in the preceding pages, and differs materially from that presented by Doctor Walcott. The appendages are not shown in their natural positions, but as if flattened nearly into a horizontal plane. The metastoma is added without any evidence for its former presence. The striking features of the appendages are the broad unsegmented exopodites which point forward all along the body, and the strong endopodites, which show practically no regional modification. Although the exopodites have a form which is especially adapted for use in swimming, their position is such as to indicate that they were not so used. The stout endopodites, on the other hand, probably performed the double function of natatory and ambulatory legs. [Illustration: 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: