PLATE 8. 



Page 

 Fig. 1. OrTHIS LE0NEN8IB ? 62 



I. Cast of ventral valvct 



Fig. 2. Orthis thiemei. ? 63 



2 Cast of the ventral valve. 



Figs. 3-8. Orthis leonensis 62 



8. A dorsal valve. 



4, 5. Casts of dorsal valves. 



6, 7. Casts of ventral valves. 



8. Exterior of a ventral valve. 



Figs. 9, 10. Orthis leucosia ? 63 



9, 10. The exterior of a dorsal valve and cast of the ventral valve. 



Figs. 11-19. Orthis impressa 60 



II, 12. Casts of ventral valves of young individuals. 



13. A dorsal valve retaining the shell. 



14. Cast of a dorsal valve, showing muscular impressions and vascular lines. 



15. Ventral valve retaining the shell. 



16-18. Casts of ventral valves of the usual forms. 



19. A cast of an unusually large specimen of a ventral valve. 



Figs. 20-29. Orthis tiooa 59 



20-23. Illustrations of the dorsal valve. 



24, 25. Casts ot the dorsal valve retaining impressions of the surface strise. 



26, 27. Ventral valves showing the mesial elevation. 



28, 29. Casts of the ventral valve, showing muscular impressions. 



Figs. 30-32. Orthis carinata 58 



80. Cast of a ventral valve of this species. 



81. A dorsal valve with the shell partly. exfoliated. 



82. A cast of the dorsal valve showing the form of the muscular impression. 



