PLATE XIII— Continued. 



Fig. 13. The dorsal side of a small internal cast. 

 " 14. Ventral side of the same, showing the fillings of the cavities in the rostral 



portion of the shell. 

 " 15. Interior of the ventral valve, as obtained from a gutta-percha mold taken 



from the specimen fig. 14, showing the character of the plate. 

 " 16. Transverse section across the valve for comparison with fig. 12. 



TrEMATIS PrNCTOSTRTATA Hall. 

 Page 246. 

 Fig. 17. Exterior of the upper valve enlarged two diamaters, showing the puncta 

 arranged in form of radiating strise. 

 " 18. Exterior of the lower valve, showing the situation of the foramen. 



T REM ATI S RUB IS Hall. 

 Page 213. 

 Fig. 19. View of the upper valve, showing its form and striae, enlarged to two diam- 

 eters. 



Trematis trcxcata Hall. 

 [Biscina truncata Hall, Pal. N. Y., vol. iv, pi. 1, fig. 15 ; pi. 2, figs. 36, 37. ] 

 Fig. 20. View of the lower valve showing the form of the foramen, enlarged three 

 diameters. 



Trematis filosa Hall. 

 [Orbieula? filosa Hall, Pal. X. T., vol. 1, p. 99, pi. 30, fig. 9.] 

 Fig. 21. View of the upper valve of a large specimen, from the Utica slate. 

 " 22. View of the lower valve showing the foramen. 



Zygospira He ad II Bill. 



Figs. 23 and 24. Dorsal and profile views of a specimen, showing the general features 

 of the species. 



Fig. 25. Outline figure showing the characters of the spire, as actually seen in a 

 specimen partially filled with crystalline matter, and cut to show the inter- 

 nal arrangement of one-half of the spire and loop. 



