No. 560] CAMBRIAN HOLOTHURIANS 507 



By exactly the same reasoning Laggania cambria is 

 assigned to a position in the same group. 



The type specimen of Maclcenzia costalis shows a 

 pleated structure which can only be interpreted as due 

 to longitudinal mesenteries, probably eight in number; 

 there appear to have been sixteen processes around the 

 mouth which probably indicate tentacles retracted before 

 preservation; the distal portion of the body resembles 

 closely the distal portion of the body in the genus 

 Edwardsia. Thus, as Mackenzia costalis presents char- 

 acters not found outside of the Zoantharia, and in that 

 group peculiar to the family Edwardsiidae, it seems 

 necessary to assign it to a position in the family Ed- 

 wardsiidae, near the genus Edwardsia. 



