NO. 3 BURGESS SHALE FOSSILS WALCOTT 35 



gnathobasic spines or lobes. It is probable, however, from their posi- 

 tion 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 dis- 

 tal joints they evidently were little used in walking. The fringed exop- 

 odites may have assisted in swimming and they undoubtedly served as 

 gills. The absence of a channel formed of gnathobases on the protop- 

 odites 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 bear- 

 ing 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 Ncolcuus^ 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 at- 

 tached to the carapace in its cephalic region only. The antennae (a), 

 mandibles (m), maxillulae (ni'), and maxillae (//;") 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 (c) 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 



^ Smithsonian Misc. Coll., vol. 67, no. 7, p. 370, 1920. 



