PLATE 100. 



Pofre 



Fig. 1-8. Spirifer arenosus. 425 



1 a, b. Dorsal and ventral views of a very young specimen, showing tlie 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 whicli the cardinal angles 



are a little protluced. 

 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. 



C. 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 forriraen partially closed by a callosity, and an excessive thickening 

 of the shell on the inside. The inner laminae of the shell are much briikim away. 



8 o. The exterior of the ventral valve of a large individual, which preserves the external 

 markings iu a good degree of perfection. 



5 b. The interior of the same, showing the large area, partially closed foramen, and dental 



lamella', while the inner lamina of the shell around the nui.scular impression is exfoliated. 



Pig. 9 - 12. Spirifer pyxidatits. 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. 



