Page:Smithsonian Miscellaneous Collections, Volume 85.djvu/125

Rh gnathobasic spines or lobes. It is probable, however, from their position and form they were of service in directing food to the mouth.

The long, flat outer joints of the mandibles may have been used in swimming as an aid to the thoracic limbs.

The endopodite of the biramous trunk appendages probably served both as a natatory and ambulatory leg, although from their delicate distal joints they evidently were little used in walking. The fringed exopodites may have assisted in swimming and they undoubtedly served as gills. The absence of a channel formed of gnathobases on the protopodites of the trunk limbs such as occurs in the Apodidae and Trilobita, and also of any known organ for seizing food, leads to the conclusion that the exopodite may have served to direct a current of water bearing food towards the mouth. The long, strong filaments attached to the segments of the exopodite are comparable with the filaments of the exopodite of the trilobite Neolenus, and the remarks on the latter should be read in this connection.

Diagrammatic restoration of the ventral appendages.—This is shown in text figure 9 and presents my interpretation of the arrangement of the appendages. The long body, with its numerous segments, is attached to the carapace in its cephalic region only. The antennae (a), mandibles (m), maxillulae (m ' ), and maxillae (m  '') are drawn in from the evidence given by many specimens, no one of which has all the limbs in place; several of the best specimens are illustrated on plate 22. The eyes (e) are somewhat theoretically placed, but their general position is known despite the displacement most of them have been subjected to. On the right side the entire series of protopodites and endopodites forming the thoracic limb are represented; the form of the inner end of the protopodite is based on indications afforded by several specimens, although usually the protopodite is crushed flat and appears to be of the same size throughout its length. On the left side the protopodite is cut away so as to show the approximate point of attachment of the proximal end of the exopodite. The latter are drawn from such specimens as are represented on figures 1-9, plate 22, and many others not illustrated. The relative position and form of the exopodite, endopodite and protopodite is shown by text figure 10.

Mode of occurrence.—Marrella splendens occurs abundantly in the compact, hard shale but there are few really fine specimens. This free-swimming, delicate little crustacean dropped down on the surface of the bottom and was speedily buried by fine mud settling over it; the mass of gradually hardening mud pressed the rounded body into a