TAB. V. 



a X 



50 



h X 



50 



c X 



50 



a X 



50 



h X 



50 



c X 



50 



d X 



50 



e X 



50 



«! X 



50 



h X 



50 



c X 



50 



d X 



50 



a X 



30 



i X 



30 



c X 



30 



d X 



30 



e X 



30 



fx 



30 



fx 



30 



fx 



30 



f"X 



30 



a X 



30 



h X 



30 



c X 



30 



d X 



30 



e X 



30 



f X 



30 



.9 X 



30 



h X 



30 



Fig. 12. Cythereis Lonsdaleiana. 



Left valve, outside . . . Chalk. Norwich. 



Right valve, outside . . „ „ 



Right valve, dorsal edge . . . „ „ 



Fig. 13. Cytliereis cornuta. 



Adult, right valve, outside . . Detritus. Charing. 



Young, left valve, outside 

 Young, perfect, dorsal aspect 

 Young, perfect, ventral aspect 

 Young, perfect, anterior aspect 



Fig. 14. Cythereis alata. 



Right valve, outside . . . Chalk. Norwich. 



Perfect, dorsal aspect 

 Perfect, ventral aspect 

 Perfect, anterior aspect 



Fig. 15. Bairdia subdeltoidea. 



Perfect, dorsal aspect . . Detritus. ChariVig. 



Perfect, anterior aspect 



Left valve, outside . . . Chalk. Gravesend. 



Right valve, outside 



Left valve, inside 



Right valve, inside 



Right valve, dorsal edge 



Right valve, anterior edge . 



Right valve, posterior edge 



Fig. 16. Bairdia Siliqua. 



Perfect, dorsal aspect . . . Detritus. Charing. 



Perfect, anterior aspect . . . „ „ 



Right valve, outside . . . Chalk. Gravesend. 



Right valve, inside . . . „ „ 



Var. a, perfect, anterior aspect . . Detritus. Charing. 

 Var. a, perfect, obliquely placed, left valve 



upwards . . . . „ „ 



Var. a, perfect, dorsal aspect . . „ „ 



Var. jS, perfect, right valve upwards . „ „ 



