400 CALIFOKNIA AIIADKMY OF SCIENCES. 



EXPLANATION OF PLATE XIX. 



Fig. I. Mactra (^Spisula) falcata Gould. Upper San Pedro series, San Pedro. Interior 



of left valve. Longitude 47 mm.; natural size. 176 



Fig. 2. Mactra californica Conrad. Uj^per San Pedro series, San Pedro. Interior of left 



valve. Longitude 39 mm.; natural size. '74 



Fig. 3. Mactra hemphilli Dai.i,. Upper San Pedro series, San Pedro. Interior of right 



valve. Longitude 124 mm; natural size. '75 



Fig. 4. Mactra exoleta Gray. Upper San Pedro series, San Pedro. Interior view, right 



valve. Longitude 57 mm.; natural size. '75 



Fig. 5. Mactra {Spisula) catilliformis Conrad. Up])er San Pedro series, San Pedro. In- 

 terior of right valve. Longitude 125 mm.; natural size. i?^ 



