PLATE 20. 



Puge 



Fig. 1, 2 & 3. Steophodonta headleyana. 185 



1 a. Interior of the ventral valve, where the surface is marked by strong I'ounded striic, and 

 the muscular area striate. The hinge, along its line of junction with the opposite valve, 

 presents a linear gioove from the termination of the crenulations to the cardinal extremi- 

 ties : creimlations extending little more than one-third the length of the hiuge-line on 

 either side of the centre. 



1 6. "Enlargement of a portion of the hinge-line and area of 1 a. 



2 a. b. Interior of the ventral valve of two specimens which present some variation in the 



form of the muscular impressions. 



2 c. Enlargement of the papillose interior .surface. 

 2 e. Enlargement of the cast of the muscular area. 



2 d. Cast of a ventral valve, showing the narrow callosity which fills the foramen. 

 2/. Enlargement of the punctate casts of striae. 



3 a. Exterior of a ventral valve, showing the peculiar interrupted striation. 

 3 b. Profile showing the concavity of the ventral valve. 



3 c. Enlargement of a portion of the striae. 



o d. Interior of a valve having similar characters to 3 a, but more deeply concave. 



3 e. Profile of 2 b, showing the convexity of the interior of the valve. 



