26 



SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. 85 



Dorsal shield. — Owing to its extreme tenuity there is usually little 

 more than a dark film on the shale that has definite outlines, and shin- 

 ing through it are traces of the digestive organs and the ventral limbs. 

 The transverse cephalic carapace recalls that of Marrella without the 

 great median spines ; it is often incurved at the center of its anterior 

 margin and laterally projects into long backward-curving, spine-like 

 extensions that are so tenuous as to suggest that the cephalic carapace 

 was formed of a delicate membrane. 



t t 



Fig. 8. — Skania iragilis Walcott. 



(X7.) Diagrammatic outline of ventral side showing the various parts as 

 interpreted from several specimens. No details of the segments of the posterior 

 dorsal shield are preserved, but the segments are clearly outlined. The intestine 

 is quite definite, also the fact that it contracted at each segment and expanded 

 into a stomach beneath the cephalic carapace. Only the proximal portion of 

 the limbs is outlined, although fragments of the distal portion are preserved on 

 one specimen. 



The thoracic portion of the dorsal shield is clearly segmented in two 

 specimens, and traces of segmentation occur in others, but usually 

 there is only a black smear on the shale with the outline of the intestine 

 showing through it. There appear to be 14 or 15 fused segments and 

 possibly a minute terminal segment or telson. None of the 29 speci- 

 mens in the collection show the outlines of the median lobe, although 

 one has a slight elevation along the line of the intestine. 



Lahrum. — Traces of a narrow doublure and small labrum have been 

 seen on two si^ecimens ; the labrum appears to have been elongate with 

 an outline similar to that of the labrum of Burgcssia. 



