NO. 3 



BURC.ESS SHALE FOSSILS WALCOTT 



II 



Exoskclctou. — The dorsal exoskeleton as seen from its dorsal side 

 is formed of a carapace and a posterior segmented shield. The true 

 cephalic carapace or shell fold is attached to the cephalic somites near 

 its anterior portion, probably as in the recent Apodidae or the asso- 

 ciated Burgcssia and also along the line of the body as far back as 

 the anterior margin of the posterior dorsal carapace. The carapace is 

 not known to have had a reflected anterior margin with a labrum at- 



NanKua conipacta W'alcott. 



c, carapace; g, digestive glands; d.g., lateral digestive glands; c, eye; Ji.c, 

 hepatic caeca ; st, stomach ; 1. 1., thoracic limb. 



(About X 5-) Diagrammatic oiithne of the digestive organs. 



tached as in Burgcssia, but it may have had. as none of the specimens 

 shows the ventral side of this part in an uninjured condition. The 

 carapace is broader than long, with a somewhat uniformly rounded 

 outline except posteriorly where it is nearly transverse ; it probably 

 had a ventral as well as dorsal membrane between which the great 

 hepatic caeca were located very much as are the shell glands in the 

 Apodidae.' For some unknown reason the anterior portion of the 

 carapace is usually distorted by being crowded back so as to wrinkle 

 and shorten it. 



The large shield of the posterior part of the exoskeleton is com- 

 posed of 14 fused segments with a narrow border. It has the ap- 

 pearance of the many-segmented pygidium of the trilobite belonging 



' In no other manner can I explain the wonderful preservation of the digestive 

 tubes and caeca. 



