PLATE XX. 



Figs. 1-7. Plcoschisma oblonga, male. 



1. Shell markings anteriorly. X 270. ' 



2. Shell markings posteriorly. X 270. 



3. Frontal organ. 270. 



4. Mandible. 105. 



5. Secondary branch of antenna. X 185. 



6. Furca. 105. 



7. Penis. Vd. Vasa deferentia. X 170. 



Figs. 8-13. Butiderma rostrata, female. 



8. Shell in side view. X 52. 



9. Edge of shell antero-dorsally, showing transparent marginal 



membrane. X 155. 



10. Frontal organ. X 155. 



11. Antenniile. X 110. 



12. Mandible. X 150. 



13. Furca. X 150. 



1156) 



