Figs. 1 - 18. 
PLATE 40. 
Spirifera mesacostalis. 
Page 
240 
1, 2. Dorsal and ventral valves of small individuals, from Ithaca, N. Y., which do not possess 
the plication in the sinus. . 
3. Dorsal valve having the cardinal extremities very much extended. This is the form origi¬ 
nally described in the Fourth District Rept., as Spiri/era acuminata. 
4. Dorsal valve, showing the character of the species. 
5. Ventral valve, having the general characters of Spirifera mucronata, but possessing the pli¬ 
cation in the mesial sinus. 
6. View of the dorsal valve having the mesial fold composed of three equal plications. 
7-9. Ventral valves presenting the ordinary characters of the species, but showing consider¬ 
able variations in the cardinal extremities. 
10. 11. Casts of ventral valves of this species. 
12, 13. Cardinal view of ventral valves, showing considerable variation in the height of area. 
Figs. 14 - 22. Spirifera mesastrialis. ... 242 
14, 15. Dorsal valves of small individuals. 
16, 17. Dorsal valves of larger individuals, having the cardinal extremities rounded or but little 
extended. 
18. Dorsal valve of a specimen of this species, having the cardinal extremities greatly extended. 
19-21. Ventral valves, showing some variation in the extent of the cardinal extremities. 
22. Enlargement of surface, showing the striated plications, and in one part the fine concentric 
striae which cancellate them. The latter feature is only seen on the most perfectly pre- 
served specimens. 
(Figs. 16, 18, 19, 21, were intended to show fine striations on the sinus, but these unfor¬ 
tunately are only shown upon the early impressions. 
