EXPLANATION OF PLATE XX. 



Ptychodera hedlefii, sp. nov. 



Fig. 7. Transverse section through the collar nerve cord, x 125. 

 ,. 8. Sagittal section through the collar canal (ccl.) of one side and 

 the first and second gill pockets (gc. 1 and 2), gp. 1 and 2 : 

 Common opening of the first and second gill sockets, spr. 1 : 

 First septal bar of gill skeleton, x 80. 



[For Reference Letters see pages 345-6.] 



