PLATE 38*. 
Spirifera medialis.. 
Page 
227 
Figs. 1 -11. 
1. Dorsal valve of a small specimen, with extended cardinal extremities and fine plications. 
,2-4. Dorsal, ventral and cardinal views of an extended form, having a high, nearly vertical, 
cardinal area and narrow deltidial opening. 
5, 6. Dorsal and cardinal views of a specimen of medium size, having fine plications, a high 
area, and extended cardinal extremities. 
7. Dorsal valve of a specimen having a triangular form, from the great extension of the hinge 
line. This is a somewhat unusual form. 
8-11. Dorsal, ventral, cardinal and profile views of a narrow ventricose form, having a semi¬ 
circular outline, and a high, almost vertical area. The surface is strongly marked with 
concentric lines of growth. 
Figs. 12-18. Spirifera medialis var. eatoni. 229 
12-15. Dorsal, profile, cardinal and front views of a specimen of medium size and ventricose 
form, characteristic of this variety. 
16-18. Dorsal, cardinal and front views of a larger and somewhat unsymmetrical specimen. 
"Figs. 19-22. Spirifera medialis var. 227 
19. Cardinal view of a ventral valve with a high vertical area. 
20. Cardinal view of a still narrower form which is somewhat distorted. The area is vertical 
and the foramen very narrow. 
21. 22. Ventral and cardinal views of an exfoliated specimen, having very fine plications and 
high area, which is somewhat receding towards the apex; from limestone in the Hamil¬ 
ton shales, at Eighteen Mile Creek, shore of Lake Erie. 
Figs. 23-32. Spirifera angusta. 230 
23, 24. Dorsal and ventral views of a young individual of a short form. 
25-27. Dorsal, ventral and cardinal views of a larger specimen, having the hinge line more 
extended. 
28-31. Dorsal, ventral, cardinal and profile views of a large specimen with extended cardinal 
extremities and a high sloping area. 
32. Ventral view of a cast of a specimen with a short hinge line and rounded front margin. The 
mucronate extensions of the hinge are so nearly solid that the filling of these portions of 
the shell is rarely preserved in casts, and owing to the loss of this part they appear to 
have a shorter hinge line. 
