410 



GEOLOGICAL SURVEY OF THE TEEKITORIES. 



!Fi6. 3R. — Actual section thronsh the head of Limiilus, showing the second pair of appendages and 

 their relations to the shell or carapace: ht, heart; lii\ liver; end, appendage homologoua with the en- 

 dopodito of Decapoda. 



Fig. 39. — Diagrammatic section through hody 

 of a hypothetical fo#m to show the possible ho- 

 mologies brtwei'u till', appendages of Limulua and 

 a trilobite; the lettering as in S"ig. 40. 



Pig. 40.— Restored section of the thorax of a 

 trilobite (Calymeu»>) after Walcott: c, carapace; 

 en, endopodite; en', exopodite, with the gills 

 on the esopodal or respiratory part of the ap- 

 pendage. 



