EXPLANATIONS OF THE PLATES. 



PLATE I. (Frontispiece.) 



VARIOUS FORMS OF DIATOMACE.E. 



Fig. 1. Actinocyclus Ralfsii. 



2. Asterolampra concinna. 



3. Heliopelta (as seen with black-ground illumination). 



4. Asteromphalus Brookeii. 



5. Aulacodiscus Oreganas. 



PLATE II. (Frontispiece). 

 echinus-spine (Original), and podura-scale (after E. Beck). 



Fig. 1. Transverse section of Spine of Echinometra heteropora. 



2. Markings on Scale of Podura, as seen by transmitted light under a 

 well-corrected l-8th inch Objective. 



3. Partial obliteration of the markings by the insinuation of moisture 

 between the Scale and the Covering-glass. 



4. Appearance of the markings, when the Scale is illuminated from 

 above by oblique light falling at right angles to them. 



5. The same, when the light falls on the Scale in the direction of the 

 markings. 



PLATE III. (p. 73). 



SMITH AND BECK'S POPULAR MICROSCOPE. 



PLATE IV. (p. 76). 



ROSS'S LARGE MICROSCOPE. 



PLATE V. (p. 78). 



POWELL AMD LEALAND's SMALLER MICROSCOPE. 



PLATE VI. (p. 79). 



POWELL AND LEALAND'S LARGE MICROSCOPE. 



PLATE VII. (p. 80). 



SMITH AND BECK'S LARGE MICROSCOPE. 



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