PLATE 99. 



Fig. 1 - 10. Spiriper arenosus. 425 



1, 2 & 3. Casts of the dorsal valve of young and medium-sized specimens. 



4 & 6. Casts of larger individuals, showing the marks of the muscular impressions. 



6 a. A specimen below the medium size, in which the marks of the muscular impression are 

 well preserved, the transverse striae being as distinct as in older shells. 



6 6. Cardinal view of the same specimen. 



7 & 8. Casts of the ventral valve of very old specimens. 



9 & 10. Casts of the dorsal valve, the latter figure presenting an unusual extension of the beak. 



