PLATE 90. 



Pagrt 

 Fig. 1-7. OrTUIS HIPPABIONYX. 407 



1. Cast of the dorsal valve of a younger specimen tlian that of the preceding plate. 



2. Cast of the ventral valve of a specimen of medium size, showing the lower part of the 



muscular impression but faintly developed. 



8. A similar specimen, which preserves a portion of the shell. 



4. An older specimen, in which the cast of the muscular imprint is perfectly preserved. 



5 & 6. The interior and the cast of a very old shell. 



V. A portion of the surface strise enlarged. * 



