PLATE XI. 



Fig. 



15. Productus umbonillatus, King. Cast (natural size) of the convex valve, showing 



impressions of the superior pedicle muscles {a), and of 



the valvular and cardinal ditto {b) conjoined. Tunstall 



Hill. 



(The letter a points too much to the left.) 



16. — — Small valve of the last specimen. 



17. — — Gutta-percha cast of the flat valve of the same, showing 



the boss or cardinal muscular fulcrum («); median 

 plate (<5); reniform impressions (/) ; impression of a 

 vein connecting the latter structures with the medio- 

 cardinal region {(/). 



(The dotted line connecting the letter a with the boss has been 

 inadvertently omitted.) 



18. — — Medio-longitudinal section : «, large valve ; (5, small valve. 



The inferior side of the prolongation (umbone) on the 

 right side of the figure represents the flattened (or 

 area-like) space on that side of the umbone adjoining 

 the hinge. 



1 9. Strophalosia Goldfussi, Miinster. Ideal medio-longitudinal section : a, area of 



large valve ; h, ditto of small valve. 



20. Strophalosia excavata, Geinitz. Ideal medio-longitudinal section. 



21. Strophalosia Morrisiana, Kinff. Ideal medio-longitudinal section. 



