PLATE 11)0. 
Tage 
Fig. 1-8. Spirifer arenostjs. 425 
1 a, b. Dorsal and ventral views of a very young specimen, showing the characters already 
described. 
2. A larger specimen. 
8 a, b. Dorsal and ventral views of a larger specimen, showing the commencement of the 
dichotomizing of the plications of the- mesial fold and sinus. 
4 a. b. Dorsal and ventral views of a specimen of medium size, in which the cardinal angles 
are a little produced. 
4 c. Profile view of the same specimen. 
4 d. Cardinal view of the same. 
4 e. Front view of the same. 
5. The interior of a dorsal valve. 
6. The interior of a ventral valve, with an area of medium width, and a slight thickening on 
each side of the muscular impression. 
7. An old shell having the foramen partially closed by a callosity, and an excessive thickening 
of the shell on the inside. The inner laminae of the shell are much broken away. 
8 a. The exterior of the ventral valve of a large individual, which preserves the external 
markings in a good degree of perfection. 
8 b. The interior of the same, showing the large area, partially closed foramen, and dental 
lamellae, while the inner lamina of the shell around the muscular impression is exfoliated. 
Fig. 9 - 12. Spirifer pyxida'tus. 428 
9 a. Dorsal view of a specimen of this species. 
9 b. Ventral view of the same. 
9 c, d. Profile and front views of the same. 
10. The interior of the ventral valve, showing the area. etc. 
12. A cast of the dorsal valve. 
