EXPLANATION OF PLATE XXXIX. 

 Unless otherwise marked, the figures on this plate are copied from Ulrich. 



Figs. 1-3. Tetradella subquadrans Ulrich. 



1. Left valve, X 20. 



2. Dorsal view of same. 



3. Posterior end view. 



4,5. Tetradella quadrilirata (Hall and Whitfield). 



4. Side view of a right valve. X 20. 



5. Posterior view of same specimen. 



6. Tetradella lunatifera (Ulrich). 



Side view of right valve, X 22, showing the usual characters. 



7. Tetradella marcliica (Krause). 

 Right valve. X 15. (After Krause.) 



S, 0. Tetradella ': digitata (Krause). 



Side views of left and right valves respectively of this doubtful 

 Tetradella. The obsolescence of the ventral margin is particularly 

 characteristic of this species. 



10. Tetradella (Kiesowia) dissecta (Krause). 

 Left valve, X 10. (After Krause.) 



11. Tetradella {Kiesowia) mamillosa (Krause). 

 Left valve, X 20. (After Krause.) 



12. Tetradella (Kiesowia) radians (Krause). 

 Left valve, X 10. (After Krause.) 



13-16. Ceratopsis chambersi (Miller). 



13. Side view of a left valve of the usual form, X 20. 



14. Ventral view of the same valve. 



15. View of inner side of more elongate right valve, X 20. 



1*;. A right valve, X 20, with the posterior median ridge divided. 

 17,18. Ceratopsis chambersi robusta Ulrich. 



Side and posterior end views of a complete carapace, X 20. 

 10,20. Ceratopsis oculif era (Hall). 



10. A left valve, X 15, showing the mushroom shaped process. <After 

 Jones. ) 



20. Dorsal view of a complete carapace, X 18. 

 21,22. Ceratopsis quadrifida (Jones). 



21. Side view of right valve, X 15. (After Jones.) 



22. Ventral edge view of same. (After Jones.) 

 330 



