PLATE XV. 
Fie. 1.—Coscinopiscus Ocunvus Iripis, by dark-ground illumi- 
nation, San Francisco, 1882; magnified 200 diameters by Tolles’s 
four-tenths-inch objective; heliostat. 
Fig. 2.—The same diatom (Coscinodiscus Oculus Iridis), New 
Orleans, 1880; magnified 300 diameters by Spencer’s one-sixth-inch 
glycerine immersion objective ; heliostat. 
