PLATE 15 



Fig. 1. Comparison of series of shells from near the surface (left) and near 

 the bottom (right) at Crockett. X 2%. 



Fig. 2. Typical shell from near surface at Crockett. X 10. 



Fig. 3. Typical shell from near bottom at Crockett. X 10. 



Fig. 4. Series of shells from Crockett of progressively greater age from left 

 to right, showing change in relative position of auricle through resorption of its 

 dorsal edge with growth. X 4. 



[324] 



