PLATE XX. 
Monterey (Middle Miocene) Diatoms. 
Fig. la. Actinoptychus undulatus Ehrenberg. 
Fig. lb. Lithodesmium cornigcrum Brim. 
Fig. lc. Dictyocha gracilis (not a diatom; nature unknown). Enlargement to 1,000 
diameters of a portion of the slide shown in PI. XIX, fig. 1. 
Fig. 2. Coscinodiscus obscurus A. S., X 1,000. From the Monterey shale of the Santa 
Maria district. 
Fig. 3. Coscinodiscus subtilis Ehrenberg, X 1,000. From the Monterey shale of the 
Santa Maria district. 
Fig. 4. Coscinodiscus robustus Grev., X 1,000. From the Monterey shale of the Santa 
Maria district. 
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