NO. 3 BURGESS SHALE FOSSILS WALCOTT 1 7 



Carapace. — Carapace semicircular, with a deep notch on the poste- 

 rior side. It appears to have had an upper (dorsal) and lower (ven- 

 tral) membrane between which the irregular ramifications of the 

 hepatic caeca were located. The shell glands, so conspicuous in the 

 Apodidae, have not been recognized in Burgcssia. 



Labrum. — The labrum is attached to the reflected anterior rim 

 (doublure) of the ventral side of the carapace and extends back 

 nearly one-third its length ; the labrum is rounded posteriorly and has 

 a shallow obliquely transverse furrow on each side just in advance of 

 its posterior margin ; it was thin, readily distorted by pressure and is 

 rarely preserved; one of the best exantples is illustrated by figure 3, 

 plate 17 ; it appears to have covered the anterior portion of the mouth. 



Eyes. — The eyes are indicated by a minute round spot on each side 

 of the dorsal median axis of the carapace and a short distance within 

 the anterior margin. 



Fig. 4. — Diagrammatic outline of a thoracic limb of Pur<jrssia. 



pr, protopodite ; en. endopodite; ex, exopodite. 



The outHne is based on the examination of many specimens aided by the known 

 form of the endopodite of Marrclla which is somewhat similar. 



Diiuensioiis.^The average length of the larger specimens is about 

 10 mm. A few are 12 and many are 6 to 8 mm. in length exclusive 

 of the long telson. The relative proportions of the carapace, thorax, 

 and abdornen are indicated by the diagrammatic restoration (text 



Cephalic appendages. — These consist of well-marked antennae ( figs. 

 3, 4) and three pairs of limbs situated between the antennae and the 

 hepato-pancreas tubes ; there is evidence that the basal or proximal 

 joints of the cephalic limbs are relatively large and the remaining 

 joints slender, but their exact position in relation to the labrum and 

 their details of form and structure are not determined. It is quite 

 probable that they represent the mandibles, maxillulae. and maxillae 

 very much as in Marrella, and I have so represented them in the 

 diagrammatic restoration of the ventral view of the species (text 



fig-3)- 



Thoracic limbs.- — The ten pairs of biramous thoracic limbs are uni- 

 form in character with the exception of the posterior pair, ^^hich are 



