PLATE 20. 
1, 2 & 8. Strophodonta headleyana. 
1 a. Interior of the ventral valve, where the surface is marked by strong rounded stria;, and 
the muscular area striate. The hinge, along its line of junction with the opposite valve, 
presents a linear groove from the termination of the crenulations to the cardinal extremi¬ 
ties : crenulations extending little more than one-third the length of the hinge-line on 
either side of the centre. 
1 b. 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. 
if. Enlargement of tire punctate casts of stria;. 
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. 
3 d. Interior of a valve having similar characters to 3 a, hut more deeply concave. 
3 e. Profile of 2 b, showing the convexity of the interior of the valve. 
