PLATE 89. 



Puge 



Fig. 1-4. Oktiiis hipparionyx. 407 



1 a. A dorsal valve which retains a small portion of the shell. The direction of the striae upon 

 the hinge-margin is very well shown. 



1 b. Profile view of the same. 



2 a. A similar cast, showing the vascular impressions upon the surface of the more prominent 



portions, and likewise towards the anterior margin. 



2 6. Profile of the same from the cardinal side. 



2 c. The interior of the upper part of the dorsal valve and brachial processes, as shown in a 

 mould from the preceding figure. 



8. A very old specimen, in which the muscular impression is extremely elongated. 

 4. The cast of a shell of medium size, preserving the muscular impression in its usual sym- 

 metrical form. 



